SemaOverload.cpp

//===--- SemaOverload.cpp - C++ Overloading -------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file provides Sema routines for C++ overloading.
//
//===----------------------------------------------------------------------===//

#include "clang/AST/ASTContext.h"
#include "clang/AST/CXXInheritance.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/DeclObjC.h"
#include "clang/AST/DependenceFlags.h"
#include "clang/AST/Expr.h"
#include "clang/AST/ExprCXX.h"
#include "clang/AST/ExprObjC.h"
#include "clang/AST/Type.h"
#include "clang/AST/TypeOrdering.h"
#include "clang/Basic/Diagnostic.h"
#include "clang/Basic/DiagnosticOptions.h"
#include "clang/Basic/OperatorKinds.h"
#include "clang/Basic/PartialDiagnostic.h"
#include "clang/Basic/SourceManager.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/Sema/EnterExpressionEvaluationContext.h"
#include "clang/Sema/Initialization.h"
#include "clang/Sema/Lookup.h"
#include "clang/Sema/Overload.h"
#include "clang/Sema/SemaInternal.h"
#include "clang/Sema/Template.h"
#include "clang/Sema/TemplateDeduction.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/Support/Casting.h"
#include <algorithm>
#include <cstdlib>
#include <optional>

using namespace clang;
using namespace sema;

using AllowedExplicit = Sema::AllowedExplicit;

static bool functionHasPassObjectSizeParams(const FunctionDecl *FD) {
  return llvm::any_of(FD->parameters(), [](const ParmVarDecl *P) {
    return P->hasAttr<PassObjectSizeAttr>();
  });
}

/// A convenience routine for creating a decayed reference to a function.
static ExprResult CreateFunctionRefExpr(
    Sema &S, FunctionDecl *Fn, NamedDecl *FoundDecl, const Expr *Base,
    bool HadMultipleCandidates, SourceLocation Loc = SourceLocation(),
    const DeclarationNameLoc &LocInfo = DeclarationNameLoc()) {
  if (S.DiagnoseUseOfDecl(FoundDecl, Loc))
    return ExprError();
  // If FoundDecl is different from Fn (such as if one is a template
  // and the other a specialization), make sure DiagnoseUseOfDecl is
  // called on both.
  // FIXME: This would be more comprehensively addressed by modifying
  // DiagnoseUseOfDecl to accept both the FoundDecl and the decl
  // being used.
  if (FoundDecl != Fn && S.DiagnoseUseOfDecl(Fn, Loc))
    return ExprError();
  DeclRefExpr *DRE = new (S.Context)
      DeclRefExpr(S.Context, Fn, false, Fn->getType(), VK_LValue, Loc, LocInfo);
  if (HadMultipleCandidates)
    DRE->setHadMultipleCandidates(true);

  S.MarkDeclRefReferenced(DRE, Base);
  if (auto *FPT = DRE->getType()->getAs<FunctionProtoType>()) {
    if (isUnresolvedExceptionSpec(FPT->getExceptionSpecType())) {
      S.ResolveExceptionSpec(Loc, FPT);
      DRE->setType(Fn->getType());
    }
  }
  return S.ImpCastExprToType(DRE, S.Context.getPointerType(DRE->getType()),
                             CK_FunctionToPointerDecay);
}

static bool IsStandardConversion(Sema &S, Expr* From, QualType ToType,
                                 bool InOverloadResolution,
                                 StandardConversionSequence &SCS,
                                 bool CStyle,
                                 bool AllowObjCWritebackConversion);

static bool IsTransparentUnionStandardConversion(Sema &S, Expr* From,
                                                 QualType &ToType,
                                                 bool InOverloadResolution,
                                                 StandardConversionSequence &SCS,
                                                 bool CStyle);
static OverloadingResult
IsUserDefinedConversion(Sema &S, Expr *From, QualType ToType,
                        UserDefinedConversionSequence& User,
                        OverloadCandidateSet& Conversions,
                        AllowedExplicit AllowExplicit,
                        bool AllowObjCConversionOnExplicit);

static ImplicitConversionSequence::CompareKind
CompareStandardConversionSequences(Sema &S, SourceLocation Loc,
                                   const StandardConversionSequence& SCS1,
                                   const StandardConversionSequence& SCS2);

static ImplicitConversionSequence::CompareKind
CompareQualificationConversions(Sema &S,
                                const StandardConversionSequence& SCS1,
                                const StandardConversionSequence& SCS2);

static ImplicitConversionSequence::CompareKind
CompareDerivedToBaseConversions(Sema &S, SourceLocation Loc,
                                const StandardConversionSequence& SCS1,
                                const StandardConversionSequence& SCS2);

/// GetConversionRank - Retrieve the implicit conversion rank
/// corresponding to the given implicit conversion kind.
ImplicitConversionRank clang::GetConversionRank(ImplicitConversionKind Kind) {
  static const ImplicitConversionRank
    Rank[] = {
    ICR_Exact_Match,
    ICR_Exact_Match,
    ICR_Exact_Match,
    ICR_Exact_Match,
    ICR_Exact_Match,
    ICR_Exact_Match,
    ICR_Promotion,
    ICR_Promotion,
    ICR_Promotion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_OCL_Scalar_Widening,
    ICR_Complex_Real_Conversion,
    ICR_Conversion,
    ICR_Conversion,
    ICR_Writeback_Conversion,
    ICR_Exact_Match, // NOTE(gbiv): This may not be completely right --
                     // it was omitted by the patch that added
                     // ICK_Zero_Event_Conversion
    ICR_Exact_Match, // NOTE(ctopper): This may not be completely right --
                     // it was omitted by the patch that added
                     // ICK_Zero_Queue_Conversion
    ICR_C_Conversion,
    ICR_C_Conversion_Extension
  };
  static_assert(std::size(Rank) == (int)ICK_Num_Conversion_Kinds);
  return Rank[(int)Kind];
}

/// GetImplicitConversionName - Return the name of this kind of
/// implicit conversion.
static const char* GetImplicitConversionName(ImplicitConversionKind Kind) {
  static const char* const Name[] = {
    "No conversion",
    "Lvalue-to-rvalue",
    "Array-to-pointer",
    "Function-to-pointer",
    "Function pointer conversion",
    "Qualification",
    "Integral promotion",
    "Floating point promotion",
    "Complex promotion",
    "Integral conversion",
    "Floating conversion",
    "Complex conversion",
    "Floating-integral conversion",
    "Pointer conversion",
    "Pointer-to-member conversion",
    "Boolean conversion",
    "Compatible-types conversion",
    "Derived-to-base conversion",
    "Vector conversion",
    "SVE Vector conversion",
    "RVV Vector conversion",
    "Vector splat",
    "Complex-real conversion",
    "Block Pointer conversion",
    "Transparent Union Conversion",
    "Writeback conversion",
    "OpenCL Zero Event Conversion",
    "OpenCL Zero Queue Conversion",
    "C specific type conversion",
    "Incompatible pointer conversion"
  };
  static_assert(std::size(Name) == (int)ICK_Num_Conversion_Kinds);
  return Name[Kind];
}

/// StandardConversionSequence - Set the standard conversion
/// sequence to the identity conversion.
void StandardConversionSequence::setAsIdentityConversion() {
  First = ICK_Identity;
  Second = ICK_Identity;
  Third = ICK_Identity;
  DeprecatedStringLiteralToCharPtr = false;
  QualificationIncludesObjCLifetime = false;
  ReferenceBinding = false;
  DirectBinding = false;
  IsLvalueReference = true;
  BindsToFunctionLvalue = false;
  BindsToRvalue = false;
  BindsImplicitObjectArgumentWithoutRefQualifier = false;
  ObjCLifetimeConversionBinding = false;
  CopyConstructor = nullptr;
}

/// getRank - Retrieve the rank of this standard conversion sequence
/// (C++ 13.3.3.1.1p3). The rank is the largest rank of each of the
/// implicit conversions.
ImplicitConversionRank StandardConversionSequence::getRank() const {
  ImplicitConversionRank Rank = ICR_Exact_Match;
  if  (GetConversionRank(First) > Rank)
    Rank = GetConversionRank(First);
  if  (GetConversionRank(Second) > Rank)
    Rank = GetConversionRank(Second);
  if  (GetConversionRank(Third) > Rank)
    Rank = GetConversionRank(Third);
  return Rank;
}

/// isPointerConversionToBool - Determines whether this conversion is
/// a conversion of a pointer or pointer-to-member to bool. This is
/// used as part of the ranking of standard conversion sequences
/// (C++ 13.3.3.2p4).
bool StandardConversionSequence::isPointerConversionToBool() const {
  // Note that FromType has not necessarily been transformed by the
  // array-to-pointer or function-to-pointer implicit conversions, so
  // check for their presence as well as checking whether FromType is
  // a pointer.
  if (getToType(1)->isBooleanType() &&
      (getFromType()->isPointerType() ||
       getFromType()->isMemberPointerType() ||
       getFromType()->isObjCObjectPointerType() ||
       getFromType()->isBlockPointerType() ||
       First == ICK_Array_To_Pointer || First == ICK_Function_To_Pointer))
    return true;

  return false;
}

/// isPointerConversionToVoidPointer - Determines whether this
/// conversion is a conversion of a pointer to a void pointer. This is
/// used as part of the ranking of standard conversion sequences (C++
/// 13.3.3.2p4).
bool
StandardConversionSequence::
isPointerConversionToVoidPointer(ASTContext& Context) const {
  QualType FromType = getFromType();
  QualType ToType = getToType(1);

  // Note that FromType has not necessarily been transformed by the
  // array-to-pointer implicit conversion, so check for its presence
  // and redo the conversion to get a pointer.
  if (First == ICK_Array_To_Pointer)
    FromType = Context.getArrayDecayedType(FromType);

  if (Second == ICK_Pointer_Conversion && FromType->isAnyPointerType())
    if (const PointerType* ToPtrType = ToType->getAs<PointerType>())
      return ToPtrType->getPointeeType()->isVoidType();

  return false;
}

/// Skip any implicit casts which could be either part of a narrowing conversion
/// or after one in an implicit conversion.
static const Expr *IgnoreNarrowingConversion(ASTContext &Ctx,
                                             const Expr *Converted) {
  // We can have cleanups wrapping the converted expression; these need to be
  // preserved so that destructors run if necessary.
  if (auto *EWC = dyn_cast<ExprWithCleanups>(Converted)) {
    Expr *Inner =
        const_cast<Expr *>(IgnoreNarrowingConversion(Ctx, EWC->getSubExpr()));
    return ExprWithCleanups::Create(Ctx, Inner, EWC->cleanupsHaveSideEffects(),
                                    EWC->getObjects());
  }

  while (auto *ICE = dyn_cast<ImplicitCastExpr>(Converted)) {
    switch (ICE->getCastKind()) {
    case CK_NoOp:
    case CK_IntegralCast:
    case CK_IntegralToBoolean:
    case CK_IntegralToFloating:
    case CK_BooleanToSignedIntegral:
    case CK_FloatingToIntegral:
    case CK_FloatingToBoolean:
    case CK_FloatingCast:
      Converted = ICE->getSubExpr();
      continue;

    default:
      return Converted;
    }
  }

  return Converted;
}

/// Check if this standard conversion sequence represents a narrowing
/// conversion, according to C++11 [dcl.init.list]p7.
///
/// \param Ctx  The AST context.
/// \param Converted  The result of applying this standard conversion sequence.
/// \param ConstantValue  If this is an NK_Constant_Narrowing conversion, the
///        value of the expression prior to the narrowing conversion.
/// \param ConstantType  If this is an NK_Constant_Narrowing conversion, the
///        type of the expression prior to the narrowing conversion.
/// \param IgnoreFloatToIntegralConversion If true type-narrowing conversions
///        from floating point types to integral types should be ignored.
NarrowingKind StandardConversionSequence::getNarrowingKind(
    ASTContext &Ctx, const Expr *Converted, APValue &ConstantValue,
    QualType &ConstantType, bool IgnoreFloatToIntegralConversion) const {
  assert(Ctx.getLangOpts().CPlusPlus && "narrowing check outside C++");

  // C++11 [dcl.init.list]p7:
  //   A narrowing conversion is an implicit conversion ...
  QualType FromType = getToType(0);
  QualType ToType = getToType(1);

  // A conversion to an enumeration type is narrowing if the conversion to
  // the underlying type is narrowing. This only arises for expressions of
  // the form 'Enum{init}'.
  if (auto *ET = ToType->getAs<EnumType>())
    ToType = ET->getDecl()->getIntegerType();

  switch (Second) {
  // 'bool' is an integral type; dispatch to the right place to handle it.
  case ICK_Boolean_Conversion:
    if (FromType->isRealFloatingType())
      goto FloatingIntegralConversion;
    if (FromType->isIntegralOrUnscopedEnumerationType())
      goto IntegralConversion;
    // -- from a pointer type or pointer-to-member type to bool, or
    return NK_Type_Narrowing;

  // -- from a floating-point type to an integer type, or
  //
  // -- from an integer type or unscoped enumeration type to a floating-point
  //    type, except where the source is a constant expression and the actual
  //    value after conversion will fit into the target type and will produce
  //    the original value when converted back to the original type, or
  case ICK_Floating_Integral:
  FloatingIntegralConversion:
    if (FromType->isRealFloatingType() && ToType->isIntegralType(Ctx)) {
      return NK_Type_Narrowing;
    } else if (FromType->isIntegralOrUnscopedEnumerationType() &&
               ToType->isRealFloatingType()) {
      if (IgnoreFloatToIntegralConversion)
        return NK_Not_Narrowing;
      const Expr *Initializer = IgnoreNarrowingConversion(Ctx, Converted);
      assert(Initializer && "Unknown conversion expression");

      // If it's value-dependent, we can't tell whether it's narrowing.
      if (Initializer->isValueDependent())
        return NK_Dependent_Narrowing;

      if (std::optional<llvm::APSInt> IntConstantValue =
              Initializer->getIntegerConstantExpr(Ctx)) {
        // Convert the integer to the floating type.
        llvm::APFloat Result(Ctx.getFloatTypeSemantics(ToType));
        Result.convertFromAPInt(*IntConstantValue, IntConstantValue->isSigned(),
                                llvm::APFloat::rmNearestTiesToEven);
        // And back.
        llvm::APSInt ConvertedValue = *IntConstantValue;
        bool ignored;
        Result.convertToInteger(ConvertedValue,
                                llvm::APFloat::rmTowardZero, &ignored);
        // If the resulting value is different, this was a narrowing conversion.
        if (*IntConstantValue != ConvertedValue) {
          ConstantValue = APValue(*IntConstantValue);
          ConstantType = Initializer->getType();
          return NK_Constant_Narrowing;
        }
      } else {
        // Variables are always narrowings.
        return NK_Variable_Narrowing;
      }
    }
    return NK_Not_Narrowing;

  // -- from long double to double or float, or from double to float, except
  //    where the source is a constant expression and the actual value after
  //    conversion is within the range of values that can be represented (even
  //    if it cannot be represented exactly), or
  case ICK_Floating_Conversion:
    if (FromType->isRealFloatingType() && ToType->isRealFloatingType() &&
        Ctx.getFloatingTypeOrder(FromType, ToType) == 1) {
      // FromType is larger than ToType.
      const Expr *Initializer = IgnoreNarrowingConversion(Ctx, Converted);

      // If it's value-dependent, we can't tell whether it's narrowing.
      if (Initializer->isValueDependent())
        return NK_Dependent_Narrowing;

      if (Initializer->isCXX11ConstantExpr(Ctx, &ConstantValue)) {
        // Constant!
        assert(ConstantValue.isFloat());
        llvm::APFloat FloatVal = ConstantValue.getFloat();
        // Convert the source value into the target type.
        bool ignored;
        llvm::APFloat::opStatus ConvertStatus = FloatVal.convert(
          Ctx.getFloatTypeSemantics(ToType),
          llvm::APFloat::rmNearestTiesToEven, &ignored);
        // If there was no overflow, the source value is within the range of
        // values that can be represented.
        if (ConvertStatus & llvm::APFloat::opOverflow) {
          ConstantType = Initializer->getType();
          return NK_Constant_Narrowing;
        }
      } else {
        return NK_Variable_Narrowing;
      }
    }
    return NK_Not_Narrowing;

  // -- from an integer type or unscoped enumeration type to an integer type
  //    that cannot represent all the values of the original type, except where
  //    the source is a constant expression and the actual value after
  //    conversion will fit into the target type and will produce the original
  //    value when converted back to the original type.
  case ICK_Integral_Conversion:
  IntegralConversion: {
    assert(FromType->isIntegralOrUnscopedEnumerationType());
    assert(ToType->isIntegralOrUnscopedEnumerationType());
    const bool FromSigned = FromType->isSignedIntegerOrEnumerationType();
    const unsigned FromWidth = Ctx.getIntWidth(FromType);
    const bool ToSigned = ToType->isSignedIntegerOrEnumerationType();
    const unsigned ToWidth = Ctx.getIntWidth(ToType);

    if (FromWidth > ToWidth ||
        (FromWidth == ToWidth && FromSigned != ToSigned) ||
        (FromSigned && !ToSigned)) {
      // Not all values of FromType can be represented in ToType.
      const Expr *Initializer = IgnoreNarrowingConversion(Ctx, Converted);

      // If it's value-dependent, we can't tell whether it's narrowing.
      if (Initializer->isValueDependent())
        return NK_Dependent_Narrowing;

      std::optional<llvm::APSInt> OptInitializerValue;
      if (!(OptInitializerValue = Initializer->getIntegerConstantExpr(Ctx))) {
        // Such conversions on variables are always narrowing.
        return NK_Variable_Narrowing;
      }
      llvm::APSInt &InitializerValue = *OptInitializerValue;
      bool Narrowing = false;
      if (FromWidth < ToWidth) {
        // Negative -> unsigned is narrowing. Otherwise, more bits is never
        // narrowing.
        if (InitializerValue.isSigned() && InitializerValue.isNegative())
          Narrowing = true;
      } else {
        // Add a bit to the InitializerValue so we don't have to worry about
        // signed vs. unsigned comparisons.
        InitializerValue = InitializerValue.extend(
          InitializerValue.getBitWidth() + 1);
        // Convert the initializer to and from the target width and signed-ness.
        llvm::APSInt ConvertedValue = InitializerValue;
        ConvertedValue = ConvertedValue.trunc(ToWidth);
        ConvertedValue.setIsSigned(ToSigned);
        ConvertedValue = ConvertedValue.extend(InitializerValue.getBitWidth());
        ConvertedValue.setIsSigned(InitializerValue.isSigned());
        // If the result is different, this was a narrowing conversion.
        if (ConvertedValue != InitializerValue)
          Narrowing = true;
      }
      if (Narrowing) {
        ConstantType = Initializer->getType();
        ConstantValue = APValue(InitializerValue);
        return NK_Constant_Narrowing;
      }
    }
    return NK_Not_Narrowing;
  }

  default:
    // Other kinds of conversions are not narrowings.
    return NK_Not_Narrowing;
  }
}

/// dump - Print this standard conversion sequence to standard
/// error. Useful for debugging overloading issues.
LLVM_DUMP_METHOD void StandardConversionSequence::dump() const {
  raw_ostream &OS = llvm::errs();
  bool PrintedSomething = false;
  if (First != ICK_Identity) {
    OS << GetImplicitConversionName(First);
    PrintedSomething = true;
  }

  if (Second != ICK_Identity) {
    if (PrintedSomething) {
      OS << " -> ";
    }
    OS << GetImplicitConversionName(Second);

    if (CopyConstructor) {
      OS << " (by copy constructor)";
    } else if (DirectBinding) {
      OS << " (direct reference binding)";
    } else if (ReferenceBinding) {
      OS << " (reference binding)";
    }
    PrintedSomething = true;
  }

  if (Third != ICK_Identity) {
    if (PrintedSomething) {
      OS << " -> ";
    }
    OS << GetImplicitConversionName(Third);
    PrintedSomething = true;
  }

  if (!PrintedSomething) {
    OS << "No conversions required";
  }
}

/// dump - Print this user-defined conversion sequence to standard
/// error. Useful for debugging overloading issues.
void UserDefinedConversionSequence::dump() const {
  raw_ostream &OS = llvm::errs();
  if (Before.First || Before.Second || Before.Third) {
    Before.dump();
    OS << " -> ";
  }
  if (ConversionFunction)
    OS << '\'' << *ConversionFunction << '\'';
  else
    OS << "aggregate initialization";
  if (After.First || After.Second || After.Third) {
    OS << " -> ";
    After.dump();
  }
}

/// dump - Print this implicit conversion sequence to standard
/// error. Useful for debugging overloading issues.
void ImplicitConversionSequence::dump() const {
  raw_ostream &OS = llvm::errs();
  if (hasInitializerListContainerType())
    OS << "Worst list element conversion: ";
  switch (ConversionKind) {
  case StandardConversion:
    OS << "Standard conversion: ";
    Standard.dump();
    break;
  case UserDefinedConversion:
    OS << "User-defined conversion: ";
    UserDefined.dump();
    break;
  case EllipsisConversion:
    OS << "Ellipsis conversion";
    break;
  case AmbiguousConversion:
    OS << "Ambiguous conversion";
    break;
  case BadConversion:
    OS << "Bad conversion";
    break;
  }

  OS << "\n";
}

void AmbiguousConversionSequence::construct() {
  new (&conversions()) ConversionSet();
}

void AmbiguousConversionSequence::destruct() {
  conversions().~ConversionSet();
}

void
AmbiguousConversionSequence::copyFrom(const AmbiguousConversionSequence &O) {
  FromTypePtr = O.FromTypePtr;
  ToTypePtr = O.ToTypePtr;
  new (&conversions()) ConversionSet(O.conversions());
}

namespace {
  // Structure used by DeductionFailureInfo to store
  // template argument information.
  struct DFIArguments {
    TemplateArgument FirstArg;
    TemplateArgument SecondArg;
  };
  // Structure used by DeductionFailureInfo to store
  // template parameter and template argument information.
  struct DFIParamWithArguments : DFIArguments {
    TemplateParameter Param;
  };
  // Structure used by DeductionFailureInfo to store template argument
  // information and the index of the problematic call argument.
  struct DFIDeducedMismatchArgs : DFIArguments {
    TemplateArgumentList *TemplateArgs;
    unsigned CallArgIndex;
  };
  // Structure used by DeductionFailureInfo to store information about
  // unsatisfied constraints.
  struct CNSInfo {
    TemplateArgumentList *TemplateArgs;
    ConstraintSatisfaction Satisfaction;
  };
}

/// Convert from Sema's representation of template deduction information
/// to the form used in overload-candidate information.
DeductionFailureInfo
clang::MakeDeductionFailureInfo(ASTContext &Context,
                                Sema::TemplateDeductionResult TDK,
                                TemplateDeductionInfo &Info) {
  DeductionFailureInfo Result;
  Result.Result = static_cast<unsigned>(TDK);
  Result.HasDiagnostic = false;
  switch (TDK) {
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_MiscellaneousDeductionFailure:
  case Sema::TDK_CUDATargetMismatch:
    Result.Data = nullptr;
    break;

  case Sema::TDK_Incomplete:
  case Sema::TDK_InvalidExplicitArguments:
    Result.Data = Info.Param.getOpaqueValue();
    break;

  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested: {
    // FIXME: Should allocate from normal heap so that we can free this later.
    auto *Saved = new (Context) DFIDeducedMismatchArgs;
    Saved->FirstArg = Info.FirstArg;
    Saved->SecondArg = Info.SecondArg;
    Saved->TemplateArgs = Info.takeSugared();
    Saved->CallArgIndex = Info.CallArgIndex;
    Result.Data = Saved;
    break;
  }

  case Sema::TDK_NonDeducedMismatch: {
    // FIXME: Should allocate from normal heap so that we can free this later.
    DFIArguments *Saved = new (Context) DFIArguments;
    Saved->FirstArg = Info.FirstArg;
    Saved->SecondArg = Info.SecondArg;
    Result.Data = Saved;
    break;
  }

  case Sema::TDK_IncompletePack:
    // FIXME: It's slightly wasteful to allocate two TemplateArguments for this.
  case Sema::TDK_Inconsistent:
  case Sema::TDK_Underqualified: {
    // FIXME: Should allocate from normal heap so that we can free this later.
    DFIParamWithArguments *Saved = new (Context) DFIParamWithArguments;
    Saved->Param = Info.Param;
    Saved->FirstArg = Info.FirstArg;
    Saved->SecondArg = Info.SecondArg;
    Result.Data = Saved;
    break;
  }

  case Sema::TDK_SubstitutionFailure:
    Result.Data = Info.takeSugared();
    if (Info.hasSFINAEDiagnostic()) {
      PartialDiagnosticAt *Diag = new (Result.Diagnostic) PartialDiagnosticAt(
          SourceLocation(), PartialDiagnostic::NullDiagnostic());
      Info.takeSFINAEDiagnostic(*Diag);
      Result.HasDiagnostic = true;
    }
    break;

  case Sema::TDK_ConstraintsNotSatisfied: {
    CNSInfo *Saved = new (Context) CNSInfo;
    Saved->TemplateArgs = Info.takeSugared();
    Saved->Satisfaction = Info.AssociatedConstraintsSatisfaction;
    Result.Data = Saved;
    break;
  }

  case Sema::TDK_Success:
  case Sema::TDK_NonDependentConversionFailure:
  case Sema::TDK_AlreadyDiagnosed:
    llvm_unreachable("not a deduction failure");
  }

  return Result;
}

void DeductionFailureInfo::Destroy() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_Success:
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_Incomplete:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_InvalidExplicitArguments:
  case Sema::TDK_CUDATargetMismatch:
  case Sema::TDK_NonDependentConversionFailure:
    break;

  case Sema::TDK_IncompletePack:
  case Sema::TDK_Inconsistent:
  case Sema::TDK_Underqualified:
  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
  case Sema::TDK_NonDeducedMismatch:
    // FIXME: Destroy the data?
    Data = nullptr;
    break;

  case Sema::TDK_SubstitutionFailure:
    // FIXME: Destroy the template argument list?
    Data = nullptr;
    if (PartialDiagnosticAt *Diag = getSFINAEDiagnostic()) {
      Diag->~PartialDiagnosticAt();
      HasDiagnostic = false;
    }
    break;

  case Sema::TDK_ConstraintsNotSatisfied:
    // FIXME: Destroy the template argument list?
    Data = nullptr;
    if (PartialDiagnosticAt *Diag = getSFINAEDiagnostic()) {
      Diag->~PartialDiagnosticAt();
      HasDiagnostic = false;
    }
    break;

  // Unhandled
  case Sema::TDK_MiscellaneousDeductionFailure:
  case Sema::TDK_AlreadyDiagnosed:
    break;
  }
}

PartialDiagnosticAt *DeductionFailureInfo::getSFINAEDiagnostic() {
  if (HasDiagnostic)
    return static_cast<PartialDiagnosticAt*>(static_cast<void*>(Diagnostic));
  return nullptr;
}

TemplateParameter DeductionFailureInfo::getTemplateParameter() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_Success:
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_SubstitutionFailure:
  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
  case Sema::TDK_NonDeducedMismatch:
  case Sema::TDK_CUDATargetMismatch:
  case Sema::TDK_NonDependentConversionFailure:
  case Sema::TDK_ConstraintsNotSatisfied:
    return TemplateParameter();

  case Sema::TDK_Incomplete:
  case Sema::TDK_InvalidExplicitArguments:
    return TemplateParameter::getFromOpaqueValue(Data);

  case Sema::TDK_IncompletePack:
  case Sema::TDK_Inconsistent:
  case Sema::TDK_Underqualified:
    return static_cast<DFIParamWithArguments*>(Data)->Param;

  // Unhandled
  case Sema::TDK_MiscellaneousDeductionFailure:
  case Sema::TDK_AlreadyDiagnosed:
    break;
  }

  return TemplateParameter();
}

TemplateArgumentList *DeductionFailureInfo::getTemplateArgumentList() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_Success:
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_Incomplete:
  case Sema::TDK_IncompletePack:
  case Sema::TDK_InvalidExplicitArguments:
  case Sema::TDK_Inconsistent:
  case Sema::TDK_Underqualified:
  case Sema::TDK_NonDeducedMismatch:
  case Sema::TDK_CUDATargetMismatch:
  case Sema::TDK_NonDependentConversionFailure:
    return nullptr;

  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
    return static_cast<DFIDeducedMismatchArgs*>(Data)->TemplateArgs;

  case Sema::TDK_SubstitutionFailure:
    return static_cast<TemplateArgumentList*>(Data);

  case Sema::TDK_ConstraintsNotSatisfied:
    return static_cast<CNSInfo*>(Data)->TemplateArgs;

  // Unhandled
  case Sema::TDK_MiscellaneousDeductionFailure:
  case Sema::TDK_AlreadyDiagnosed:
    break;
  }

  return nullptr;
}

const TemplateArgument *DeductionFailureInfo::getFirstArg() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_Success:
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_Incomplete:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_InvalidExplicitArguments:
  case Sema::TDK_SubstitutionFailure:
  case Sema::TDK_CUDATargetMismatch:
  case Sema::TDK_NonDependentConversionFailure:
  case Sema::TDK_ConstraintsNotSatisfied:
    return nullptr;

  case Sema::TDK_IncompletePack:
  case Sema::TDK_Inconsistent:
  case Sema::TDK_Underqualified:
  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
  case Sema::TDK_NonDeducedMismatch:
    return &static_cast<DFIArguments*>(Data)->FirstArg;

  // Unhandled
  case Sema::TDK_MiscellaneousDeductionFailure:
  case Sema::TDK_AlreadyDiagnosed:
    break;
  }

  return nullptr;
}

const TemplateArgument *DeductionFailureInfo::getSecondArg() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_Success:
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_Incomplete:
  case Sema::TDK_IncompletePack:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_InvalidExplicitArguments:
  case Sema::TDK_SubstitutionFailure:
  case Sema::TDK_CUDATargetMismatch:
  case Sema::TDK_NonDependentConversionFailure:
  case Sema::TDK_ConstraintsNotSatisfied:
    return nullptr;

  case Sema::TDK_Inconsistent:
  case Sema::TDK_Underqualified:
  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
  case Sema::TDK_NonDeducedMismatch:
    return &static_cast<DFIArguments*>(Data)->SecondArg;

  // Unhandled
  case Sema::TDK_MiscellaneousDeductionFailure:
  case Sema::TDK_AlreadyDiagnosed:
    break;
  }

  return nullptr;
}

std::optional<unsigned> DeductionFailureInfo::getCallArgIndex() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
    return static_cast<DFIDeducedMismatchArgs*>(Data)->CallArgIndex;

  default:
    return std::nullopt;
  }
}

static bool FunctionsCorrespond(ASTContext &Ctx, const FunctionDecl *X,
                                const FunctionDecl *Y) {
  if (!X || !Y)
    return false;
  if (X->getNumParams() != Y->getNumParams())
    return false;
  for (unsigned I = 0; I < X->getNumParams(); ++I)
    if (!Ctx.hasSameUnqualifiedType(X->getParamDecl(I)->getType(),
                                    Y->getParamDecl(I)->getType()))
      return false;
  if (auto *FTX = X->getDescribedFunctionTemplate()) {
    auto *FTY = Y->getDescribedFunctionTemplate();
    if (!FTY)
      return false;
    if (!Ctx.isSameTemplateParameterList(FTX->getTemplateParameters(),
                                         FTY->getTemplateParameters()))
      return false;
  }
  return true;
}

static bool shouldAddReversedEqEq(Sema &S, SourceLocation OpLoc,
                                  Expr *FirstOperand, FunctionDecl *EqFD) {
  assert(EqFD->getOverloadedOperator() ==
         OverloadedOperatorKind::OO_EqualEqual);
  // C++2a [over.match.oper]p4:
  // A non-template function or function template F named operator== is a
  // rewrite target with first operand o unless a search for the name operator!=
  // in the scope S from the instantiation context of the operator expression
  // finds a function or function template that would correspond
  // ([basic.scope.scope]) to F if its name were operator==, where S is the
  // scope of the class type of o if F is a class member, and the namespace
  // scope of which F is a member otherwise. A function template specialization
  // named operator== is a rewrite target if its function template is a rewrite
  // target.
  DeclarationName NotEqOp = S.Context.DeclarationNames.getCXXOperatorName(
      OverloadedOperatorKind::OO_ExclaimEqual);
  if (isa<CXXMethodDecl>(EqFD)) {
    // If F is a class member, search scope is class type of first operand.
    QualType RHS = FirstOperand->getType();
    auto *RHSRec = RHS->getAs<RecordType>();
    if (!RHSRec)
      return true;
    LookupResult Members(S, NotEqOp, OpLoc,
                         Sema::LookupNameKind::LookupMemberName);
    S.LookupQualifiedName(Members, RHSRec->getDecl());
    Members.suppressDiagnostics();
    for (NamedDecl *Op : Members)
      if (FunctionsCorrespond(S.Context, EqFD, Op->getAsFunction()))
        return false;
    return true;
  }
  // Otherwise the search scope is the namespace scope of which F is a member.
  LookupResult NonMembers(S, NotEqOp, OpLoc,
                          Sema::LookupNameKind::LookupOperatorName);
  S.LookupName(NonMembers,
               S.getScopeForContext(EqFD->getEnclosingNamespaceContext()));
  NonMembers.suppressDiagnostics();
  for (NamedDecl *Op : NonMembers) {
    auto *FD = Op->getAsFunction();
    if(auto* UD = dyn_cast<UsingShadowDecl>(Op))
      FD = UD->getUnderlyingDecl()->getAsFunction();
    if (FunctionsCorrespond(S.Context, EqFD, FD) &&
        declaresSameEntity(cast<Decl>(EqFD->getDeclContext()),
                           cast<Decl>(Op->getDeclContext())))
      return false;
  }
  return true;
}

bool OverloadCandidateSet::OperatorRewriteInfo::allowsReversed(
    OverloadedOperatorKind Op) {
  if (!AllowRewrittenCandidates)
    return false;
  return Op == OO_EqualEqual || Op == OO_Spaceship;
}

bool OverloadCandidateSet::OperatorRewriteInfo::shouldAddReversed(
    Sema &S, ArrayRef<Expr *> OriginalArgs, FunctionDecl *FD) {
  auto Op = FD->getOverloadedOperator();
  if (!allowsReversed(Op))
    return false;
  if (Op == OverloadedOperatorKind::OO_EqualEqual) {
    assert(OriginalArgs.size() == 2);
    if (!shouldAddReversedEqEq(
            S, OpLoc, /*FirstOperand in reversed args*/ OriginalArgs[1], FD))
      return false;
  }
  // Don't bother adding a reversed candidate that can never be a better
  // match than the non-reversed version.
  return FD->getNumParams() != 2 ||
         !S.Context.hasSameUnqualifiedType(FD->getParamDecl(0)->getType(),
                                           FD->getParamDecl(1)->getType()) ||
         FD->hasAttr<EnableIfAttr>();
}

void OverloadCandidateSet::destroyCandidates() {
  for (iterator i = begin(), e = end(); i != e; ++i) {
    for (auto &C : i->Conversions)
      C.~ImplicitConversionSequence();
    if (!i->Viable && i->FailureKind == ovl_fail_bad_deduction)
      i->DeductionFailure.Destroy();
  }
}

void OverloadCandidateSet::clear(CandidateSetKind CSK) {
  destroyCandidates();
  SlabAllocator.Reset();
  NumInlineBytesUsed = 0;
  Candidates.clear();
  Functions.clear();
  Kind = CSK;
}

namespace {
  class UnbridgedCastsSet {
    struct Entry {
      Expr **Addr;
      Expr *Saved;
    };
    SmallVector<Entry, 2> Entries;

  public:
    void save(Sema &S, Expr *&E) {
      assert(E->hasPlaceholderType(BuiltinType::ARCUnbridgedCast));
      Entry entry = { &E, E };
      Entries.push_back(entry);
      E = S.stripARCUnbridgedCast(E);
    }

    void restore() {
      for (SmallVectorImpl<Entry>::iterator
             i = Entries.begin(), e = Entries.end(); i != e; ++i)
        *i->Addr = i->Saved;
    }
  };
}

/// checkPlaceholderForOverload - Do any interesting placeholder-like
/// preprocessing on the given expression.
///
/// \param unbridgedCasts a collection to which to add unbridged casts;
///   without this, they will be immediately diagnosed as errors
///
/// Return true on unrecoverable error.
static bool
checkPlaceholderForOverload(Sema &S, Expr *&E,
                            UnbridgedCastsSet *unbridgedCasts = nullptr) {
  if (const BuiltinType *placeholder =  E->getType()->getAsPlaceholderType()) {
    // We can't handle overloaded expressions here because overload
    // resolution might reasonably tweak them.
    if (placeholder->getKind() == BuiltinType::Overload) return false;

    // If the context potentially accepts unbridged ARC casts, strip
    // the unbridged cast and add it to the collection for later restoration.
    if (placeholder->getKind() == BuiltinType::ARCUnbridgedCast &&
        unbridgedCasts) {
      unbridgedCasts->save(S, E);
      return false;
    }

    // Go ahead and check everything else.
    ExprResult result = S.CheckPlaceholderExpr(E);
    if (result.isInvalid())
      return true;

    E = result.get();
    return false;
  }

  // Nothing to do.
  return false;
}

/// checkArgPlaceholdersForOverload - Check a set of call operands for
/// placeholders.
static bool checkArgPlaceholdersForOverload(Sema &S, MultiExprArg Args,
                                            UnbridgedCastsSet &unbridged) {
  for (unsigned i = 0, e = Args.size(); i != e; ++i)
    if (checkPlaceholderForOverload(S, Args[i], &unbridged))
      return true;

  return false;
}

/// Determine whether the given New declaration is an overload of the
/// declarations in Old. This routine returns Ovl_Match or Ovl_NonFunction if
/// New and Old cannot be overloaded, e.g., if New has the same signature as
/// some function in Old (C++ 1.3.10) or if the Old declarations aren't
/// functions (or function templates) at all. When it does return Ovl_Match or
/// Ovl_NonFunction, MatchedDecl will point to the decl that New cannot be
/// overloaded with. This decl may be a UsingShadowDecl on top of the underlying
/// declaration.
///
/// Example: Given the following input:
///
///   void f(int, float); // #1
///   void f(int, int); // #2
///   int f(int, int); // #3
///
/// When we process #1, there is no previous declaration of "f", so IsOverload
/// will not be used.
///
/// When we process #2, Old contains only the FunctionDecl for #1. By comparing
/// the parameter types, we see that #1 and #2 are overloaded (since they have
/// different signatures), so this routine returns Ovl_Overload; MatchedDecl is
/// unchanged.
///
/// When we process #3, Old is an overload set containing #1 and #2. We compare
/// the signatures of #3 to #1 (they're overloaded, so we do nothing) and then
/// #3 to #2. Since the signatures of #3 and #2 are identical (return types of
/// functions are not part of the signature), IsOverload returns Ovl_Match and
/// MatchedDecl will be set to point to the FunctionDecl for #2.
///
/// 'NewIsUsingShadowDecl' indicates that 'New' is being introduced into a class
/// by a using declaration. The rules for whether to hide shadow declarations
/// ignore some properties which otherwise figure into a function template's
/// signature.
Sema::OverloadKind
Sema::CheckOverload(Scope *S, FunctionDecl *New, const LookupResult &Old,
                    NamedDecl *&Match, bool NewIsUsingDecl) {
  for (LookupResult::iterator I = Old.begin(), E = Old.end();
         I != E; ++I) {
    NamedDecl *OldD = *I;

    bool OldIsUsingDecl = false;
    if (isa<UsingShadowDecl>(OldD)) {
      OldIsUsingDecl = true;

      // We can always introduce two using declarations into the same
      // context, even if they have identical signatures.
      if (NewIsUsingDecl) continue;

      OldD = cast<UsingShadowDecl>(OldD)->getTargetDecl();
    }

    // A using-declaration does not conflict with another declaration
    // if one of them is hidden.
    if ((OldIsUsingDecl || NewIsUsingDecl) && !isVisible(*I))
      continue;

    // If either declaration was introduced by a using declaration,
    // we'll need to use slightly different rules for matching.
    // Essentially, these rules are the normal rules, except that
    // function templates hide function templates with different
    // return types or template parameter lists.
    bool UseMemberUsingDeclRules =
      (OldIsUsingDecl || NewIsUsingDecl) && CurContext->isRecord() &&
      !New->getFriendObjectKind();

    if (FunctionDecl *OldF = OldD->getAsFunction()) {
      if (!IsOverload(New, OldF, UseMemberUsingDeclRules)) {
        if (UseMemberUsingDeclRules && OldIsUsingDecl) {
          HideUsingShadowDecl(S, cast<UsingShadowDecl>(*I));
          continue;
        }

        if (!isa<FunctionTemplateDecl>(OldD) &&
            !shouldLinkPossiblyHiddenDecl(*I, New))
          continue;

        Match = *I;
        return Ovl_Match;
      }

      // Builtins that have custom typechecking or have a reference should
      // not be overloadable or redeclarable.
      if (!getASTContext().canBuiltinBeRedeclared(OldF)) {
        Match = *I;
        return Ovl_NonFunction;
      }
    } else if (isa<UsingDecl>(OldD) || isa<UsingPackDecl>(OldD)) {
      // We can overload with these, which can show up when doing
      // redeclaration checks for UsingDecls.
      assert(Old.getLookupKind() == LookupUsingDeclName);
    } else if (isa<TagDecl>(OldD)) {
      // We can always overload with tags by hiding them.
    } else if (auto *UUD = dyn_cast<UnresolvedUsingValueDecl>(OldD)) {
      // Optimistically assume that an unresolved using decl will
      // overload; if it doesn't, we'll have to diagnose during
      // template instantiation.
      //
      // Exception: if the scope is dependent and this is not a class
      // member, the using declaration can only introduce an enumerator.
      if (UUD->getQualifier()->isDependent() && !UUD->isCXXClassMember()) {
        Match = *I;
        return Ovl_NonFunction;
      }
    } else {
      // (C++ 13p1):
      //   Only function declarations can be overloaded; object and type
      //   declarations cannot be overloaded.
      Match = *I;
      return Ovl_NonFunction;
    }
  }

  // C++ [temp.friend]p1:
  //   For a friend function declaration that is not a template declaration:
  //    -- if the name of the friend is a qualified or unqualified template-id,
  //       [...], otherwise
  //    -- if the name of the friend is a qualified-id and a matching
  //       non-template function is found in the specified class or namespace,
  //       the friend declaration refers to that function, otherwise,
  //    -- if the name of the friend is a qualified-id and a matching function
  //       template is found in the specified class or namespace, the friend
  //       declaration refers to the deduced specialization of that function
  //       template, otherwise
  //    -- the name shall be an unqualified-id [...]
  // If we get here for a qualified friend declaration, we've just reached the
  // third bullet. If the type of the friend is dependent, skip this lookup
  // until instantiation.
  if (New->getFriendObjectKind() && New->getQualifier() &&
      !New->getDescribedFunctionTemplate() &&
      !New->getDependentSpecializationInfo() &&
      !New->getType()->isDependentType()) {
    LookupResult TemplateSpecResult(LookupResult::Temporary, Old);
    TemplateSpecResult.addAllDecls(Old);
    if (CheckFunctionTemplateSpecialization(New, nullptr, TemplateSpecResult,
                                            /*QualifiedFriend*/true)) {
      New->setInvalidDecl();
      return Ovl_Overload;
    }

    Match = TemplateSpecResult.getAsSingle<FunctionDecl>();
    return Ovl_Match;
  }

  return Ovl_Overload;
}

bool Sema::IsOverload(FunctionDecl *New, FunctionDecl *Old,
                      bool UseMemberUsingDeclRules, bool ConsiderCudaAttrs,
                      bool ConsiderRequiresClauses) {
  // C++ [basic.start.main]p2: This function shall not be overloaded.
  if (New->isMain())
    return false;

  // MSVCRT user defined entry points cannot be overloaded.
  if (New->isMSVCRTEntryPoint())
    return false;

  FunctionTemplateDecl *OldTemplate = Old->getDescribedFunctionTemplate();
  FunctionTemplateDecl *NewTemplate = New->getDescribedFunctionTemplate();

  // C++ [temp.fct]p2:
  //   A function template can be overloaded with other function templates
  //   and with normal (non-template) functions.
  if ((OldTemplate == nullptr) != (NewTemplate == nullptr))
    return true;

  // Is the function New an overload of the function Old?
  QualType OldQType = Context.getCanonicalType(Old->getType());
  QualType NewQType = Context.getCanonicalType(New->getType());

  // Compare the signatures (C++ 1.3.10) of the two functions to
  // determine whether they are overloads. If we find any mismatch
  // in the signature, they are overloads.

  // If either of these functions is a K&R-style function (no
  // prototype), then we consider them to have matching signatures.
  if (isa<FunctionNoProtoType>(OldQType.getTypePtr()) ||
      isa<FunctionNoProtoType>(NewQType.getTypePtr()))
    return false;

  const FunctionProtoType *OldType = cast<FunctionProtoType>(OldQType);
  const FunctionProtoType *NewType = cast<FunctionProtoType>(NewQType);

  // The signature of a function includes the types of its
  // parameters (C++ 1.3.10), which includes the presence or absence
  // of the ellipsis; see C++ DR 357).
  if (OldQType != NewQType &&
      (OldType->getNumParams() != NewType->getNumParams() ||
       OldType->isVariadic() != NewType->isVariadic() ||
       !FunctionParamTypesAreEqual(OldType, NewType)))
    return true;

  // For member-like friends, the enclosing class is part of the signature.
  if ((New->isMemberLikeConstrainedFriend() ||
       Old->isMemberLikeConstrainedFriend()) &&
      !New->getLexicalDeclContext()->Equals(Old->getLexicalDeclContext()))
    return true;

  if (NewTemplate) {
    // C++ [temp.over.link]p4:
    //   The signature of a function template consists of its function
    //   signature, its return type and its template parameter list. The names
    //   of the template parameters are significant only for establishing the
    //   relationship between the template parameters and the rest of the
    //   signature.
    //
    // We check the return type and template parameter lists for function
    // templates first; the remaining checks follow.
    bool SameTemplateParameterList = TemplateParameterListsAreEqual(
        NewTemplate->getTemplateParameters(),
        OldTemplate->getTemplateParameters(), false, TPL_TemplateMatch);
    bool SameReturnType = Context.hasSameType(Old->getDeclaredReturnType(),
                                              New->getDeclaredReturnType());
    // FIXME(GH58571): Match template parameter list even for non-constrained
    // template heads. This currently ensures that the code prior to C++20 is
    // not newly broken.
    bool ConstraintsInTemplateHead =
        NewTemplate->getTemplateParameters()->hasAssociatedConstraints() ||
        OldTemplate->getTemplateParameters()->hasAssociatedConstraints();
    // C++ [namespace.udecl]p11:
    //   The set of declarations named by a using-declarator that inhabits a
    //   class C does not include member functions and member function
    //   templates of a base class that "correspond" to (and thus would
    //   conflict with) a declaration of a function or function template in
    //   C.
    // Comparing return types is not required for the "correspond" check to
    // decide whether a member introduced by a shadow declaration is hidden.
    if (UseMemberUsingDeclRules && ConstraintsInTemplateHead &&
        !SameTemplateParameterList)
      return true;
    if (!UseMemberUsingDeclRules &&
        (!SameTemplateParameterList || !SameReturnType))
      return true;
  }

  if (ConsiderRequiresClauses) {
    Expr *NewRC = New->getTrailingRequiresClause(),
         *OldRC = Old->getTrailingRequiresClause();
    if ((NewRC != nullptr) != (OldRC != nullptr))
      return true;

    if (NewRC && !AreConstraintExpressionsEqual(Old, OldRC, New, NewRC))
        return true;
  }

  // If the function is a class member, its signature includes the
  // cv-qualifiers (if any) and ref-qualifier (if any) on the function itself.
  //
  // As part of this, also check whether one of the member functions
  // is static, in which case they are not overloads (C++
  // 13.1p2). While not part of the definition of the signature,
  // this check is important to determine whether these functions
  // can be overloaded.
  CXXMethodDecl *OldMethod = dyn_cast<CXXMethodDecl>(Old);
  CXXMethodDecl *NewMethod = dyn_cast<CXXMethodDecl>(New);
  if (OldMethod && NewMethod &&
      !OldMethod->isStatic() && !NewMethod->isStatic()) {
    if (OldMethod->getRefQualifier() != NewMethod->getRefQualifier()) {
      if (!UseMemberUsingDeclRules &&
          (OldMethod->getRefQualifier() == RQ_None ||
           NewMethod->getRefQualifier() == RQ_None)) {
        // C++20 [over.load]p2:
        //   - Member function declarations with the same name, the same
        //     parameter-type-list, and the same trailing requires-clause (if
        //     any), as well as member function template declarations with the
        //     same name, the same parameter-type-list, the same trailing
        //     requires-clause (if any), and the same template-head, cannot be
        //     overloaded if any of them, but not all, have a ref-qualifier.
        Diag(NewMethod->getLocation(), diag::err_ref_qualifier_overload)
            << NewMethod->getRefQualifier() << OldMethod->getRefQualifier();
        Diag(OldMethod->getLocation(), diag::note_previous_declaration);
      }
      return true;
    }

    // We may not have applied the implicit const for a constexpr member
    // function yet (because we haven't yet resolved whether this is a static
    // or non-static member function). Add it now, on the assumption that this
    // is a redeclaration of OldMethod.
    auto OldQuals = OldMethod->getMethodQualifiers();
    auto NewQuals = NewMethod->getMethodQualifiers();
    if (!getLangOpts().CPlusPlus14 && NewMethod->isConstexpr() &&
        !isa<CXXConstructorDecl>(NewMethod))
      NewQuals.addConst();
    // We do not allow overloading based off of '__restrict'.
    OldQuals.removeRestrict();
    NewQuals.removeRestrict();
    if (OldQuals != NewQuals)
      return true;
  }

  // Though pass_object_size is placed on parameters and takes an argument, we
  // consider it to be a function-level modifier for the sake of function
  // identity. Either the function has one or more parameters with
  // pass_object_size or it doesn't.
  if (functionHasPassObjectSizeParams(New) !=
      functionHasPassObjectSizeParams(Old))
    return true;

  // enable_if attributes are an order-sensitive part of the signature.
  for (specific_attr_iterator<EnableIfAttr>
         NewI = New->specific_attr_begin<EnableIfAttr>(),
         NewE = New->specific_attr_end<EnableIfAttr>(),
         OldI = Old->specific_attr_begin<EnableIfAttr>(),
         OldE = Old->specific_attr_end<EnableIfAttr>();
       NewI != NewE || OldI != OldE; ++NewI, ++OldI) {
    if (NewI == NewE || OldI == OldE)
      return true;
    llvm::FoldingSetNodeID NewID, OldID;
    NewI->getCond()->Profile(NewID, Context, true);
    OldI->getCond()->Profile(OldID, Context, true);
    if (NewID != OldID)
      return true;
  }

  if (getLangOpts().CUDA && ConsiderCudaAttrs) {
    // Don't allow overloading of destructors.  (In theory we could, but it
    // would be a giant change to clang.)
    if (!isa<CXXDestructorDecl>(New)) {
      CUDAFunctionTarget NewTarget = IdentifyCUDATarget(New),
                         OldTarget = IdentifyCUDATarget(Old);
      if (NewTarget != CFT_InvalidTarget) {
        assert((OldTarget != CFT_InvalidTarget) &&
               "Unexpected invalid target.");

        // Allow overloading of functions with same signature and different CUDA
        // target attributes.
        if (NewTarget != OldTarget)
          return true;
      }
    }
  }

  // The signatures match; this is not an overload.
  return false;
}

/// Tries a user-defined conversion from From to ToType.
///
/// Produces an implicit conversion sequence for when a standard conversion
/// is not an option. See TryImplicitConversion for more information.
static ImplicitConversionSequence
TryUserDefinedConversion(Sema &S, Expr *From, QualType ToType,
                         bool SuppressUserConversions,
                         AllowedExplicit AllowExplicit,
                         bool InOverloadResolution,
                         bool CStyle,
                         bool AllowObjCWritebackConversion,
                         bool AllowObjCConversionOnExplicit) {
  ImplicitConversionSequence ICS;

  if (SuppressUserConversions) {
    // We're not in the case above, so there is no conversion that
    // we can perform.
    ICS.setBad(BadConversionSequence::no_conversion, From, ToType);
    return ICS;
  }

  // Attempt user-defined conversion.
  OverloadCandidateSet Conversions(From->getExprLoc(),
                                   OverloadCandidateSet::CSK_Normal);
  switch (IsUserDefinedConversion(S, From, ToType, ICS.UserDefined,
                                  Conversions, AllowExplicit,
                                  AllowObjCConversionOnExplicit)) {
  case OR_Success:
  case OR_Deleted:
    ICS.setUserDefined();
    // C++ [over.ics.user]p4:
    //   A conversion of an expression of class type to the same class
    //   type is given Exact Match rank, and a conversion of an
    //   expression of class type to a base class of that type is
    //   given Conversion rank, in spite of the fact that a copy
    //   constructor (i.e., a user-defined conversion function) is
    //   called for those cases.
    if (CXXConstructorDecl *Constructor
          = dyn_cast<CXXConstructorDecl>(ICS.UserDefined.ConversionFunction)) {
      QualType FromCanon
        = S.Context.getCanonicalType(From->getType().getUnqualifiedType());
      QualType ToCanon
        = S.Context.getCanonicalType(ToType).getUnqualifiedType();
      if (Constructor->isCopyConstructor() &&
          (FromCanon == ToCanon ||
           S.IsDerivedFrom(From->getBeginLoc(), FromCanon, ToCanon))) {
        // Turn this into a "standard" conversion sequence, so that it
        // gets ranked with standard conversion sequences.
        DeclAccessPair Found = ICS.UserDefined.FoundConversionFunction;
        ICS.setStandard();
        ICS.Standard.setAsIdentityConversion();
        ICS.Standard.setFromType(From->getType());
        ICS.Standard.setAllToTypes(ToType);
        ICS.Standard.CopyConstructor = Constructor;
        ICS.Standard.FoundCopyConstructor = Found;
        if (ToCanon != FromCanon)
          ICS.Standard.Second = ICK_Derived_To_Base;
      }
    }
    break;

  case OR_Ambiguous:
    ICS.setAmbiguous();
    ICS.Ambiguous.setFromType(From->getType());
    ICS.Ambiguous.setToType(ToType);
    for (OverloadCandidateSet::iterator Cand = Conversions.begin();
         Cand != Conversions.end(); ++Cand)
      if (Cand->Best)
        ICS.Ambiguous.addConversion(Cand->FoundDecl, Cand->Function);
    break;

    // Fall through.
  case OR_No_Viable_Function:
    ICS.setBad(BadConversionSequence::no_conversion, From, ToType);
    break;
  }

  return ICS;
}

/// TryImplicitConversion - Attempt to perform an implicit conversion
/// from the given expression (Expr) to the given type (ToType). This
/// function returns an implicit conversion sequence that can be used
/// to perform the initialization. Given
///
///   void f(float f);
///   void g(int i) { f(i); }
///
/// this routine would produce an implicit conversion sequence to
/// describe the initialization of f from i, which will be a standard
/// conversion sequence containing an lvalue-to-rvalue conversion (C++
/// 4.1) followed by a floating-integral conversion (C++ 4.9).
//
/// Note that this routine only determines how the conversion can be
/// performed; it does not actually perform the conversion. As such,
/// it will not produce any diagnostics if no conversion is available,
/// but will instead return an implicit conversion sequence of kind
/// "BadConversion".
///
/// If @p SuppressUserConversions, then user-defined conversions are
/// not permitted.
/// If @p AllowExplicit, then explicit user-defined conversions are
/// permitted.
///
/// \param AllowObjCWritebackConversion Whether we allow the Objective-C
/// writeback conversion, which allows __autoreleasing id* parameters to
/// be initialized with __strong id* or __weak id* arguments.
static ImplicitConversionSequence
TryImplicitConversion(Sema &S, Expr *From, QualType ToType,
                      bool SuppressUserConversions,
                      AllowedExplicit AllowExplicit,
                      bool InOverloadResolution,
                      bool CStyle,
                      bool AllowObjCWritebackConversion,
                      bool AllowObjCConversionOnExplicit) {
  ImplicitConversionSequence ICS;
  if (IsStandardConversion(S, From, ToType, InOverloadResolution,
                           ICS.Standard, CStyle, AllowObjCWritebackConversion)){
    ICS.setStandard();
    return ICS;
  }

  if (!S.getLangOpts().CPlusPlus) {
    ICS.setBad(BadConversionSequence::no_conversion, From, ToType);
    return ICS;
  }

  // C++ [over.ics.user]p4:
  //   A conversion of an expression of class type to the same class
  //   type is given Exact Match rank, and a conversion of an
  //   expression of class type to a base class of that type is
  //   given Conversion rank, in spite of the fact that a copy/move
  //   constructor (i.e., a user-defined conversion function) is
  //   called for those cases.
  QualType FromType = From->getType();
  if (ToType->getAs<RecordType>() && FromType->getAs<RecordType>() &&
      (S.Context.hasSameUnqualifiedType(FromType, ToType) ||
       S.IsDerivedFrom(From->getBeginLoc(), FromType, ToType))) {
    ICS.setStandard();
    ICS.Standard.setAsIdentityConversion();
    ICS.Standard.setFromType(FromType);
    ICS.Standard.setAllToTypes(ToType);

    // We don't actually check at this point whether there is a valid
    // copy/move constructor, since overloading just assumes that it
    // exists. When we actually perform initialization, we'll find the
    // appropriate constructor to copy the returned object, if needed.
    ICS.Standard.CopyConstructor = nullptr;

    // Determine whether this is considered a derived-to-base conversion.
    if (!S.Context.hasSameUnqualifiedType(FromType, ToType))
      ICS.Standard.Second = ICK_Derived_To_Base;

    return ICS;
  }

  return TryUserDefinedConversion(S, From, ToType, SuppressUserConversions,
                                  AllowExplicit, InOverloadResolution, CStyle,
                                  AllowObjCWritebackConversion,
                                  AllowObjCConversionOnExplicit);
}

ImplicitConversionSequence
Sema::TryImplicitConversion(Expr *From, QualType ToType,
                            bool SuppressUserConversions,
                            AllowedExplicit AllowExplicit,
                            bool InOverloadResolution,
                            bool CStyle,
                            bool AllowObjCWritebackConversion) {
  return ::TryImplicitConversion(*this, From, ToType, SuppressUserConversions,
                                 AllowExplicit, InOverloadResolution, CStyle,
                                 AllowObjCWritebackConversion,
                                 /*AllowObjCConversionOnExplicit=*/false);
}

/// PerformImplicitConversion - Perform an implicit conversion of the
/// expression From to the type ToType. Returns the
/// converted expression. Flavor is the kind of conversion we're
/// performing, used in the error message. If @p AllowExplicit,
/// explicit user-defined conversions are permitted.
ExprResult Sema::PerformImplicitConversion(Expr *From, QualType ToType,
                                           AssignmentAction Action,
                                           bool AllowExplicit) {
  if (checkPlaceholderForOverload(*this, From))
    return ExprError();

  // Objective-C ARC: Determine whether we will allow the writeback conversion.
  bool AllowObjCWritebackConversion
    = getLangOpts().ObjCAutoRefCount &&
      (Action == AA_Passing || Action == AA_Sending);
  if (getLangOpts().ObjC)
    CheckObjCBridgeRelatedConversions(From->getBeginLoc(), ToType,
                                      From->getType(), From);
  ImplicitConversionSequence ICS = ::TryImplicitConversion(
      *this, From, ToType,
      /*SuppressUserConversions=*/false,
      AllowExplicit ? AllowedExplicit::All : AllowedExplicit::None,
      /*InOverloadResolution=*/false,
      /*CStyle=*/false, AllowObjCWritebackConversion,
      /*AllowObjCConversionOnExplicit=*/false);
  return PerformImplicitConversion(From, ToType, ICS, Action);
}

/// Determine whether the conversion from FromType to ToType is a valid
/// conversion that strips "noexcept" or "noreturn" off the nested function
/// type.
bool Sema::IsFunctionConversion(QualType FromType, QualType ToType,
                                QualType &ResultTy) {
  if (Context.hasSameUnqualifiedType(FromType, ToType))
    return false;

  // Permit the conversion F(t __attribute__((noreturn))) -> F(t)
  //                    or F(t noexcept) -> F(t)
  // where F adds one of the following at most once:
  //   - a pointer
  //   - a member pointer
  //   - a block pointer
  // Changes here need matching changes in FindCompositePointerType.
  CanQualType CanTo = Context.getCanonicalType(ToType);
  CanQualType CanFrom = Context.getCanonicalType(FromType);
  Type::TypeClass TyClass = CanTo->getTypeClass();
  if (TyClass != CanFrom->getTypeClass()) return false;
  if (TyClass != Type::FunctionProto && TyClass != Type::FunctionNoProto) {
    if (TyClass == Type::Pointer) {
      CanTo = CanTo.castAs<PointerType>()->getPointeeType();
      CanFrom = CanFrom.castAs<PointerType>()->getPointeeType();
    } else if (TyClass == Type::BlockPointer) {
      CanTo = CanTo.castAs<BlockPointerType>()->getPointeeType();
      CanFrom = CanFrom.castAs<BlockPointerType>()->getPointeeType();
    } else if (TyClass == Type::MemberPointer) {
      auto ToMPT = CanTo.castAs<MemberPointerType>();
      auto FromMPT = CanFrom.castAs<MemberPointerType>();
      // A function pointer conversion cannot change the class of the function.
      if (ToMPT->getClass() != FromMPT->getClass())
        return false;
      CanTo = ToMPT->getPointeeType();
      CanFrom = FromMPT->getPointeeType();
    } else {
      return false;
    }

    TyClass = CanTo->getTypeClass();
    if (TyClass != CanFrom->getTypeClass()) return false;
    if (TyClass != Type::FunctionProto && TyClass != Type::FunctionNoProto)
      return false;
  }

  const auto *FromFn = cast<FunctionType>(CanFrom);
  FunctionType::ExtInfo FromEInfo = FromFn->getExtInfo();

  const auto *ToFn = cast<FunctionType>(CanTo);
  FunctionType::ExtInfo ToEInfo = ToFn->getExtInfo();

  bool Changed = false;

  // Drop 'noreturn' if not present in target type.
  if (FromEInfo.getNoReturn() && !ToEInfo.getNoReturn()) {
    FromFn = Context.adjustFunctionType(FromFn, FromEInfo.withNoReturn(false));
    Changed = true;
  }

  // Drop 'noexcept' if not present in target type.
  if (const auto *FromFPT = dyn_cast<FunctionProtoType>(FromFn)) {
    const auto *ToFPT = cast<FunctionProtoType>(ToFn);
    if (FromFPT->isNothrow() && !ToFPT->isNothrow()) {
      FromFn = cast<FunctionType>(
          Context.getFunctionTypeWithExceptionSpec(QualType(FromFPT, 0),
                                                   EST_None)
                 .getTypePtr());
      Changed = true;
    }

    // Convert FromFPT's ExtParameterInfo if necessary. The conversion is valid
    // only if the ExtParameterInfo lists of the two function prototypes can be
    // merged and the merged list is identical to ToFPT's ExtParameterInfo list.
    SmallVector<FunctionProtoType::ExtParameterInfo, 4> NewParamInfos;
    bool CanUseToFPT, CanUseFromFPT;
    if (Context.mergeExtParameterInfo(ToFPT, FromFPT, CanUseToFPT,
                                      CanUseFromFPT, NewParamInfos) &&
        CanUseToFPT && !CanUseFromFPT) {
      FunctionProtoType::ExtProtoInfo ExtInfo = FromFPT->getExtProtoInfo();
      ExtInfo.ExtParameterInfos =
          NewParamInfos.empty() ? nullptr : NewParamInfos.data();
      QualType QT = Context.getFunctionType(FromFPT->getReturnType(),
                                            FromFPT->getParamTypes(), ExtInfo);
      FromFn = QT->getAs<FunctionType>();
      Changed = true;
    }
  }

  if (!Changed)
    return false;

  assert(QualType(FromFn, 0).isCanonical());
  if (QualType(FromFn, 0) != CanTo) return false;

  ResultTy = ToType;
  return true;
}

/// Determine whether the conversion from FromType to ToType is a valid
/// vector conversion.
///
/// \param ICK Will be set to the vector conversion kind, if this is a vector
/// conversion.
static bool IsVectorConversion(Sema &S, QualType FromType, QualType ToType,
                               ImplicitConversionKind &ICK, Expr *From,
                               bool InOverloadResolution, bool CStyle) {
  // We need at least one of these types to be a vector type to have a vector
  // conversion.
  if (!ToType->isVectorType() && !FromType->isVectorType())
    return false;

  // Identical types require no conversions.
  if (S.Context.hasSameUnqualifiedType(FromType, ToType))
    return false;

  // There are no conversions between extended vector types, only identity.
  if (ToType->isExtVectorType()) {
    // There are no conversions between extended vector types other than the
    // identity conversion.
    if (FromType->isExtVectorType())
      return false;

    // Vector splat from any arithmetic type to a vector.
    if (FromType->isArithmeticType()) {
      ICK = ICK_Vector_Splat;
      return true;
    }
  }

  if (ToType->isSVESizelessBuiltinType() ||
      FromType->isSVESizelessBuiltinType())
    if (S.Context.areCompatibleSveTypes(FromType, ToType) ||
        S.Context.areLaxCompatibleSveTypes(FromType, ToType)) {
      ICK = ICK_SVE_Vector_Conversion;
      return true;
    }

  if (ToType->isRVVSizelessBuiltinType() ||
      FromType->isRVVSizelessBuiltinType())
    if (S.Context.areCompatibleRVVTypes(FromType, ToType) ||
        S.Context.areLaxCompatibleRVVTypes(FromType, ToType)) {
      ICK = ICK_RVV_Vector_Conversion;
      return true;
    }

  // We can perform the conversion between vector types in the following cases:
  // 1)vector types are equivalent AltiVec and GCC vector types
  // 2)lax vector conversions are permitted and the vector types are of the
  //   same size
  // 3)the destination type does not have the ARM MVE strict-polymorphism
  //   attribute, which inhibits lax vector conversion for overload resolution
  //   only
  if (ToType->isVectorType() && FromType->isVectorType()) {
    if (S.Context.areCompatibleVectorTypes(FromType, ToType) ||
        (S.isLaxVectorConversion(FromType, ToType) &&
         !ToType->hasAttr(attr::ArmMveStrictPolymorphism))) {
      if (S.getASTContext().getTargetInfo().getTriple().isPPC() &&
          S.isLaxVectorConversion(FromType, ToType) &&
          S.anyAltivecTypes(FromType, ToType) &&
          !S.Context.areCompatibleVectorTypes(FromType, ToType) &&
          !InOverloadResolution && !CStyle) {
        S.Diag(From->getBeginLoc(), diag::warn_deprecated_lax_vec_conv_all)
            << FromType << ToType;
      }
      ICK = ICK_Vector_Conversion;
      return true;
    }
  }

  return false;
}

static bool tryAtomicConversion(Sema &S, Expr *From, QualType ToType,
                                bool InOverloadResolution,
                                StandardConversionSequence &SCS,
                                bool CStyle);

/// IsStandardConversion - Determines whether there is a standard
/// conversion sequence (C++ [conv], C++ [over.ics.scs]) from the
/// expression From to the type ToType. Standard conversion sequences
/// only consider non-class types; for conversions that involve class
/// types, use TryImplicitConversion. If a conversion exists, SCS will
/// contain the standard conversion sequence required to perform this
/// conversion and this routine will return true. Otherwise, this
/// routine will return false and the value of SCS is unspecified.
static bool IsStandardConversion(Sema &S, Expr* From, QualType ToType,
                                 bool InOverloadResolution,
                                 StandardConversionSequence &SCS,
                                 bool CStyle,
                                 bool AllowObjCWritebackConversion) {
  QualType FromType = From->getType();

  // Standard conversions (C++ [conv])
  SCS.setAsIdentityConversion();
  SCS.IncompatibleObjC = false;
  SCS.setFromType(FromType);
  SCS.CopyConstructor = nullptr;

  // There are no standard conversions for class types in C++, so
  // abort early. When overloading in C, however, we do permit them.
  if (S.getLangOpts().CPlusPlus &&
      (FromType->isRecordType() || ToType->isRecordType()))
    return false;

  // The first conversion can be an lvalue-to-rvalue conversion,
  // array-to-pointer conversion, or function-to-pointer conversion
  // (C++ 4p1).

  if (FromType == S.Context.OverloadTy) {
    DeclAccessPair AccessPair;
    if (FunctionDecl *Fn
          = S.ResolveAddressOfOverloadedFunction(From, ToType, false,
                                                 AccessPair)) {
      // We were able to resolve the address of the overloaded function,
      // so we can convert to the type of that function.
      FromType = Fn->getType();
      SCS.setFromType(FromType);

      // we can sometimes resolve &foo<int> regardless of ToType, so check
      // if the type matches (identity) or we are converting to bool
      if (!S.Context.hasSameUnqualifiedType(
                      S.ExtractUnqualifiedFunctionType(ToType), FromType)) {
        QualType resultTy;
        // if the function type matches except for [[noreturn]], it's ok
        if (!S.IsFunctionConversion(FromType,
              S.ExtractUnqualifiedFunctionType(ToType), resultTy))
          // otherwise, only a boolean conversion is standard
          if (!ToType->isBooleanType())
            return false;
      }

      // Check if the "from" expression is taking the address of an overloaded
      // function and recompute the FromType accordingly. Take advantage of the
      // fact that non-static member functions *must* have such an address-of
      // expression.
      CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn);
      if (Method && !Method->isStatic()) {
        assert(isa<UnaryOperator>(From->IgnoreParens()) &&
               "Non-unary operator on non-static member address");
        assert(cast<UnaryOperator>(From->IgnoreParens())->getOpcode()
               == UO_AddrOf &&
               "Non-address-of operator on non-static member address");
        const Type *ClassType
          = S.Context.getTypeDeclType(Method->getParent()).getTypePtr();
        FromType = S.Context.getMemberPointerType(FromType, ClassType);
      } else if (isa<UnaryOperator>(From->IgnoreParens())) {
        assert(cast<UnaryOperator>(From->IgnoreParens())->getOpcode() ==
               UO_AddrOf &&
               "Non-address-of operator for overloaded function expression");
        FromType = S.Context.getPointerType(FromType);
      }
    } else {
      return false;
    }
  }
  // Lvalue-to-rvalue conversion (C++11 4.1):
  //   A glvalue (3.10) of a non-function, non-array type T can
  //   be converted to a prvalue.
  bool argIsLValue = From->isGLValue();
  if (argIsLValue &&
      !FromType->isFunctionType() && !FromType->isArrayType() &&
      S.Context.getCanonicalType(FromType) != S.Context.OverloadTy) {
    SCS.First = ICK_Lvalue_To_Rvalue;

    // C11 6.3.2.1p2:
    //   ... if the lvalue has atomic type, the value has the non-atomic version
    //   of the type of the lvalue ...
    if (const AtomicType *Atomic = FromType->getAs<AtomicType>())
      FromType = Atomic->getValueType();

    // If T is a non-class type, the type of the rvalue is the
    // cv-unqualified version of T. Otherwise, the type of the rvalue
    // is T (C++ 4.1p1). C++ can't get here with class types; in C, we
    // just strip the qualifiers because they don't matter.
    FromType = FromType.getUnqualifiedType();
  } else if (FromType->isArrayType()) {
    // Array-to-pointer conversion (C++ 4.2)
    SCS.First = ICK_Array_To_Pointer;

    // An lvalue or rvalue of type "array of N T" or "array of unknown
    // bound of T" can be converted to an rvalue of type "pointer to
    // T" (C++ 4.2p1).
    FromType = S.Context.getArrayDecayedType(FromType);

    if (S.IsStringLiteralToNonConstPointerConversion(From, ToType)) {
      // This conversion is deprecated in C++03 (D.4)
      SCS.DeprecatedStringLiteralToCharPtr = true;

      // For the purpose of ranking in overload resolution
      // (13.3.3.1.1), this conversion is considered an
      // array-to-pointer conversion followed by a qualification
      // conversion (4.4). (C++ 4.2p2)
      SCS.Second = ICK_Identity;
      SCS.Third = ICK_Qualification;
      SCS.QualificationIncludesObjCLifetime = false;
      SCS.setAllToTypes(FromType);
      return true;
    }
  } else if (FromType->isFunctionType() && argIsLValue) {
    // Function-to-pointer conversion (C++ 4.3).
    SCS.First = ICK_Function_To_Pointer;

    if (auto *DRE = dyn_cast<DeclRefExpr>(From->IgnoreParenCasts()))
      if (auto *FD = dyn_cast<FunctionDecl>(DRE->getDecl()))
        if (!S.checkAddressOfFunctionIsAvailable(FD))
          return false;

    // An lvalue of function type T can be converted to an rvalue of
    // type "pointer to T." The result is a pointer to the
    // function. (C++ 4.3p1).
    FromType = S.Context.getPointerType(FromType);
  } else {
    // We don't require any conversions for the first step.
    SCS.First = ICK_Identity;
  }
  SCS.setToType(0, FromType);

  // The second conversion can be an integral promotion, floating
  // point promotion, integral conversion, floating point conversion,
  // floating-integral conversion, pointer conversion,
  // pointer-to-member conversion, or boolean conversion (C++ 4p1).
  // For overloading in C, this can also be a "compatible-type"
  // conversion.
  bool IncompatibleObjC = false;
  ImplicitConversionKind SecondICK = ICK_Identity;
  if (S.Context.hasSameUnqualifiedType(FromType, ToType)) {
    // The unqualified versions of the types are the same: there's no
    // conversion to do.
    SCS.Second = ICK_Identity;
  } else if (S.IsIntegralPromotion(From, FromType, ToType)) {
    // Integral promotion (C++ 4.5).
    SCS.Second = ICK_Integral_Promotion;
    FromType = ToType.getUnqualifiedType();
  } else if (S.IsFloatingPointPromotion(FromType, ToType)) {
    // Floating point promotion (C++ 4.6).
    SCS.Second = ICK_Floating_Promotion;
    FromType = ToType.getUnqualifiedType();
  } else if (S.IsComplexPromotion(FromType, ToType)) {
    // Complex promotion (Clang extension)
    SCS.Second = ICK_Complex_Promotion;
    FromType = ToType.getUnqualifiedType();
  } else if (ToType->isBooleanType() &&
             (FromType->isArithmeticType() ||
              FromType->isAnyPointerType() ||
              FromType->isBlockPointerType() ||
              FromType->isMemberPointerType())) {
    // Boolean conversions (C++ 4.12).
    SCS.Second = ICK_Boolean_Conversion;
    FromType = S.Context.BoolTy;
  } else if (FromType->isIntegralOrUnscopedEnumerationType() &&
             ToType->isIntegralType(S.Context)) {
    // Integral conversions (C++ 4.7).
    SCS.Second = ICK_Integral_Conversion;
    FromType = ToType.getUnqualifiedType();
  } else if (FromType->isAnyComplexType() && ToType->isAnyComplexType()) {
    // Complex conversions (C99 6.3.1.6)
    SCS.Second = ICK_Complex_Conversion;
    FromType = ToType.getUnqualifiedType();
  } else if ((FromType->isAnyComplexType() && ToType->isArithmeticType()) ||
             (ToType->isAnyComplexType() && FromType->isArithmeticType())) {
    // Complex-real conversions (C99 6.3.1.7)
    SCS.Second = ICK_Complex_Real;
    FromType = ToType.getUnqualifiedType();
  } else if (FromType->isRealFloatingType() && ToType->isRealFloatingType()) {
    // FIXME: disable conversions between long double, __ibm128 and __float128
    // if their representation is different until there is back end support
    // We of course allow this conversion if long double is really double.

    // Conversions between bfloat and other floats are not permitted.
    if (FromType == S.Context.BFloat16Ty || ToType == S.Context.BFloat16Ty)
      return false;

    // Conversions between IEEE-quad and IBM-extended semantics are not
    // permitted.
    const llvm::fltSemantics &FromSem =
        S.Context.getFloatTypeSemantics(FromType);
    const llvm::fltSemantics &ToSem = S.Context.getFloatTypeSemantics(ToType);
    if ((&FromSem == &llvm::APFloat::PPCDoubleDouble() &&
         &ToSem == &llvm::APFloat::IEEEquad()) ||
        (&FromSem == &llvm::APFloat::IEEEquad() &&
         &ToSem == &llvm::APFloat::PPCDoubleDouble()))
      return false;

    // Floating point conversions (C++ 4.8).
    SCS.Second = ICK_Floating_Conversion;
    FromType = ToType.getUnqualifiedType();
  } else if ((FromType->isRealFloatingType() &&
              ToType->isIntegralType(S.Context)) ||
             (FromType->isIntegralOrUnscopedEnumerationType() &&
              ToType->isRealFloatingType())) {
    // Conversions between bfloat and int are not permitted.
    if (FromType->isBFloat16Type() || ToType->isBFloat16Type())
      return false;

    // Floating-integral conversions (C++ 4.9).
    SCS.Second = ICK_Floating_Integral;
    FromType = ToType.getUnqualifiedType();
  } else if (S.IsBlockPointerConversion(FromType, ToType, FromType)) {
    SCS.Second = ICK_Block_Pointer_Conversion;
  } else if (AllowObjCWritebackConversion &&
             S.isObjCWritebackConversion(FromType, ToType, FromType)) {
    SCS.Second = ICK_Writeback_Conversion;
  } else if (S.IsPointerConversion(From, FromType, ToType, InOverloadResolution,
                                   FromType, IncompatibleObjC)) {
    // Pointer conversions (C++ 4.10).
    SCS.Second = ICK_Pointer_Conversion;
    SCS.IncompatibleObjC = IncompatibleObjC;
    FromType = FromType.getUnqualifiedType();
  } else if (S.IsMemberPointerConversion(From, FromType, ToType,
                                         InOverloadResolution, FromType)) {
    // Pointer to member conversions (4.11).
    SCS.Second = ICK_Pointer_Member;
  } else if (IsVectorConversion(S, FromType, ToType, SecondICK, From,
                                InOverloadResolution, CStyle)) {
    SCS.Second = SecondICK;
    FromType = ToType.getUnqualifiedType();
  } else if (!S.getLangOpts().CPlusPlus &&
             S.Context.typesAreCompatible(ToType, FromType)) {
    // Compatible conversions (Clang extension for C function overloading)
    SCS.Second = ICK_Compatible_Conversion;
    FromType = ToType.getUnqualifiedType();
  } else if (IsTransparentUnionStandardConversion(S, From, ToType,
                                             InOverloadResolution,
                                             SCS, CStyle)) {
    SCS.Second = ICK_TransparentUnionConversion;
    FromType = ToType;
  } else if (tryAtomicConversion(S, From, ToType, InOverloadResolution, SCS,
                                 CStyle)) {
    // tryAtomicConversion has updated the standard conversion sequence
    // appropriately.
    return true;
  } else if (ToType->isEventT() &&
             From->isIntegerConstantExpr(S.getASTContext()) &&
             From->EvaluateKnownConstInt(S.getASTContext()) == 0) {
    SCS.Second = ICK_Zero_Event_Conversion;
    FromType = ToType;
  } else if (ToType->isQueueT() &&
             From->isIntegerConstantExpr(S.getASTContext()) &&
             (From->EvaluateKnownConstInt(S.getASTContext()) == 0)) {
    SCS.Second = ICK_Zero_Queue_Conversion;
    FromType = ToType;
  } else if (ToType->isSamplerT() &&
             From->isIntegerConstantExpr(S.getASTContext())) {
    SCS.Second = ICK_Compatible_Conversion;
    FromType = ToType;
  } else {
    // No second conversion required.
    SCS.Second = ICK_Identity;
  }
  SCS.setToType(1, FromType);

  // The third conversion can be a function pointer conversion or a
  // qualification conversion (C++ [conv.fctptr], [conv.qual]).
  bool ObjCLifetimeConversion;
  if (S.IsFunctionConversion(FromType, ToType, FromType)) {
    // Function pointer conversions (removing 'noexcept') including removal of
    // 'noreturn' (Clang extension).
    SCS.Third = ICK_Function_Conversion;
  } else if (S.IsQualificationConversion(FromType, ToType, CStyle,
                                         ObjCLifetimeConversion)) {
    SCS.Third = ICK_Qualification;
    SCS.QualificationIncludesObjCLifetime = ObjCLifetimeConversion;
    FromType = ToType;
  } else {
    // No conversion required
    SCS.Third = ICK_Identity;
  }

  // C++ [over.best.ics]p6:
  //   [...] Any difference in top-level cv-qualification is
  //   subsumed by the initialization itself and does not constitute
  //   a conversion. [...]
  QualType CanonFrom = S.Context.getCanonicalType(FromType);
  QualType CanonTo = S.Context.getCanonicalType(ToType);
  if (CanonFrom.getLocalUnqualifiedType()
                                     == CanonTo.getLocalUnqualifiedType() &&
      CanonFrom.getLocalQualifiers() != CanonTo.getLocalQualifiers()) {
    FromType = ToType;
    CanonFrom = CanonTo;
  }

  SCS.setToType(2, FromType);

  if (CanonFrom == CanonTo)
    return true;

  // If we have not converted the argument type to the parameter type,
  // this is a bad conversion sequence, unless we're resolving an overload in C.
  if (S.getLangOpts().CPlusPlus || !InOverloadResolution)
    return false;

  ExprResult ER = ExprResult{From};
  Sema::AssignConvertType Conv =
      S.CheckSingleAssignmentConstraints(ToType, ER,
                                         /*Diagnose=*/false,
                                         /*DiagnoseCFAudited=*/false,
                                         /*ConvertRHS=*/false);
  ImplicitConversionKind SecondConv;
  switch (Conv) {
  case Sema::Compatible:
    SecondConv = ICK_C_Only_Conversion;
    break;
  // For our purposes, discarding qualifiers is just as bad as using an
  // incompatible pointer. Note that an IncompatiblePointer conversion can drop
  // qualifiers, as well.
  case Sema::CompatiblePointerDiscardsQualifiers:
  case Sema::IncompatiblePointer:
  case Sema::IncompatiblePointerSign:
    SecondConv = ICK_Incompatible_Pointer_Conversion;
    break;
  default:
    return false;
  }

  // First can only be an lvalue conversion, so we pretend that this was the
  // second conversion. First should already be valid from earlier in the
  // function.
  SCS.Second = SecondConv;
  SCS.setToType(1, ToType);

  // Third is Identity, because Second should rank us worse than any other
  // conversion. This could also be ICK_Qualification, but it's simpler to just
  // lump everything in with the second conversion, and we don't gain anything
  // from making this ICK_Qualification.
  SCS.Third = ICK_Identity;
  SCS.setToType(2, ToType);
  return true;
}

static bool
IsTransparentUnionStandardConversion(Sema &S, Expr* From,
                                     QualType &ToType,
                                     bool InOverloadResolution,
                                     StandardConversionSequence &SCS,
                                     bool CStyle) {

  const RecordType *UT = ToType->getAsUnionType();
  if (!UT || !UT->getDecl()->hasAttr<TransparentUnionAttr>())
    return false;
  // The field to initialize within the transparent union.
  RecordDecl *UD = UT->getDecl();
  // It's compatible if the expression matches any of the fields.
  for (const auto *it : UD->fields()) {
    if (IsStandardConversion(S, From, it->getType(), InOverloadResolution, SCS,
                             CStyle, /*AllowObjCWritebackConversion=*/false)) {
      ToType = it->getType();
      return true;
    }
  }
  return false;
}

/// IsIntegralPromotion - Determines whether the conversion from the
/// expression From (whose potentially-adjusted type is FromType) to
/// ToType is an integral promotion (C++ 4.5). If so, returns true and
/// sets PromotedType to the promoted type.
bool Sema::IsIntegralPromotion(Expr *From, QualType FromType, QualType ToType) {
  const BuiltinType *To = ToType->getAs<BuiltinType>();
  // All integers are built-in.
  if (!To) {
    return false;
  }

  // An rvalue of type char, signed char, unsigned char, short int, or
  // unsigned short int can be converted to an rvalue of type int if
  // int can represent all the values of the source type; otherwise,
  // the source rvalue can be converted to an rvalue of type unsigned
  // int (C++ 4.5p1).
  if (Context.isPromotableIntegerType(FromType) && !FromType->isBooleanType() &&
      !FromType->isEnumeralType()) {
    if ( // We can promote any signed, promotable integer type to an int
        (FromType->isSignedIntegerType() ||
         // We can promote any unsigned integer type whose size is
         // less than int to an int.
         Context.getTypeSize(FromType) < Context.getTypeSize(ToType))) {
      return To->getKind() == BuiltinType::Int;
    }

    return To->getKind() == BuiltinType::UInt;
  }

  // C++11 [conv.prom]p3:
  //   A prvalue of an unscoped enumeration type whose underlying type is not
  //   fixed (7.2) can be converted to an rvalue a prvalue of the first of the
  //   following types that can represent all the values of the enumeration
  //   (i.e., the values in the range bmin to bmax as described in 7.2): int,
  //   unsigned int, long int, unsigned long int, long long int, or unsigned
  //   long long int. If none of the types in that list can represent all the
  //   values of the enumeration, an rvalue a prvalue of an unscoped enumeration
  //   type can be converted to an rvalue a prvalue of the extended integer type
  //   with lowest integer conversion rank (4.13) greater than the rank of long
  //   long in which all the values of the enumeration can be represented. If
  //   there are two such extended types, the signed one is chosen.
  // C++11 [conv.prom]p4:
  //   A prvalue of an unscoped enumeration type whose underlying type is fixed
  //   can be converted to a prvalue of its underlying type. Moreover, if
  //   integral promotion can be applied to its underlying type, a prvalue of an
  //   unscoped enumeration type whose underlying type is fixed can also be
  //   converted to a prvalue of the promoted underlying type.
  if (const EnumType *FromEnumType = FromType->getAs<EnumType>()) {
    // C++0x 7.2p9: Note that this implicit enum to int conversion is not
    // provided for a scoped enumeration.
    if (FromEnumType->getDecl()->isScoped())
      return false;

    // We can perform an integral promotion to the underlying type of the enum,
    // even if that's not the promoted type. Note that the check for promoting
    // the underlying type is based on the type alone, and does not consider
    // the bitfield-ness of the actual source expression.
    if (FromEnumType->getDecl()->isFixed()) {
      QualType Underlying = FromEnumType->getDecl()->getIntegerType();
      return Context.hasSameUnqualifiedType(Underlying, ToType) ||
             IsIntegralPromotion(nullptr, Underlying, ToType);
    }

    // We have already pre-calculated the promotion type, so this is trivial.
    if (ToType->isIntegerType() &&
        isCompleteType(From->getBeginLoc(), FromType))
      return Context.hasSameUnqualifiedType(
          ToType, FromEnumType->getDecl()->getPromotionType());

    // C++ [conv.prom]p5:
    //   If the bit-field has an enumerated type, it is treated as any other
    //   value of that type for promotion purposes.
    //
    // ... so do not fall through into the bit-field checks below in C++.
    if (getLangOpts().CPlusPlus)
      return false;
  }

  // C++0x [conv.prom]p2:
  //   A prvalue of type char16_t, char32_t, or wchar_t (3.9.1) can be converted
  //   to an rvalue a prvalue of the first of the following types that can
  //   represent all the values of its underlying type: int, unsigned int,
  //   long int, unsigned long int, long long int, or unsigned long long int.
  //   If none of the types in that list can represent all the values of its
  //   underlying type, an rvalue a prvalue of type char16_t, char32_t,
  //   or wchar_t can be converted to an rvalue a prvalue of its underlying
  //   type.
  if (FromType->isAnyCharacterType() && !FromType->isCharType() &&
      ToType->isIntegerType()) {
    // Determine whether the type we're converting from is signed or
    // unsigned.
    bool FromIsSigned = FromType->isSignedIntegerType();
    uint64_t FromSize = Context.getTypeSize(FromType);

    // The types we'll try to promote to, in the appropriate
    // order. Try each of these types.
    QualType PromoteTypes[6] = {
      Context.IntTy, Context.UnsignedIntTy,
      Context.LongTy, Context.UnsignedLongTy ,
      Context.LongLongTy, Context.UnsignedLongLongTy
    };
    for (int Idx = 0; Idx < 6; ++Idx) {
      uint64_t ToSize = Context.getTypeSize(PromoteTypes[Idx]);
      if (FromSize < ToSize ||
          (FromSize == ToSize &&
           FromIsSigned == PromoteTypes[Idx]->isSignedIntegerType())) {
        // We found the type that we can promote to. If this is the
        // type we wanted, we have a promotion. Otherwise, no
        // promotion.
        return Context.hasSameUnqualifiedType(ToType, PromoteTypes[Idx]);
      }
    }
  }

  // An rvalue for an integral bit-field (9.6) can be converted to an
  // rvalue of type int if int can represent all the values of the
  // bit-field; otherwise, it can be converted to unsigned int if
  // unsigned int can represent all the values of the bit-field. If
  // the bit-field is larger yet, no integral promotion applies to
  // it. If the bit-field has an enumerated type, it is treated as any
  // other value of that type for promotion purposes (C++ 4.5p3).
  // FIXME: We should delay checking of bit-fields until we actually perform the
  // conversion.
  //
  // FIXME: In C, only bit-fields of types _Bool, int, or unsigned int may be
  // promoted, per C11 6.3.1.1/2. We promote all bit-fields (including enum
  // bit-fields and those whose underlying type is larger than int) for GCC
  // compatibility.
  if (From) {
    if (FieldDecl *MemberDecl = From->getSourceBitField()) {
      std::optional<llvm::APSInt> BitWidth;
      if (FromType->isIntegralType(Context) &&
          (BitWidth =
               MemberDecl->getBitWidth()->getIntegerConstantExpr(Context))) {
        llvm::APSInt ToSize(BitWidth->getBitWidth(), BitWidth->isUnsigned());
        ToSize = Context.getTypeSize(ToType);

        // Are we promoting to an int from a bitfield that fits in an int?
        if (*BitWidth < ToSize ||
            (FromType->isSignedIntegerType() && *BitWidth <= ToSize)) {
          return To->getKind() == BuiltinType::Int;
        }

        // Are we promoting to an unsigned int from an unsigned bitfield
        // that fits into an unsigned int?
        if (FromType->isUnsignedIntegerType() && *BitWidth <= ToSize) {
          return To->getKind() == BuiltinType::UInt;
        }

        return false;
      }
    }
  }

  // An rvalue of type bool can be converted to an rvalue of type int,
  // with false becoming zero and true becoming one (C++ 4.5p4).
  if (FromType->isBooleanType() && To->getKind() == BuiltinType::Int) {
    return true;
  }

  return false;
}

/// IsFloatingPointPromotion - Determines whether the conversion from
/// FromType to ToType is a floating point promotion (C++ 4.6). If so,
/// returns true and sets PromotedType to the promoted type.
bool Sema::IsFloatingPointPromotion(QualType FromType, QualType ToType) {
  if (const BuiltinType *FromBuiltin = FromType->getAs<BuiltinType>())
    if (const BuiltinType *ToBuiltin = ToType->getAs<BuiltinType>()) {
      /// An rvalue of type float can be converted to an rvalue of type
      /// double. (C++ 4.6p1).
      if (FromBuiltin->getKind() == BuiltinType::Float &&
          ToBuiltin->getKind() == BuiltinType::Double)
        return true;

      // C99 6.3.1.5p1:
      //   When a float is promoted to double or long double, or a
      //   double is promoted to long double [...].
      if (!getLangOpts().CPlusPlus &&
          (FromBuiltin->getKind() == BuiltinType::Float ||
           FromBuiltin->getKind() == BuiltinType::Double) &&
          (ToBuiltin->getKind() == BuiltinType::LongDouble ||
           ToBuiltin->getKind() == BuiltinType::Float128 ||
           ToBuiltin->getKind() == BuiltinType::Ibm128))
        return true;

      // Half can be promoted to float.
      if (!getLangOpts().NativeHalfType &&
           FromBuiltin->getKind() == BuiltinType::Half &&
          ToBuiltin->getKind() == BuiltinType::Float)
        return true;
    }

  return false;
}

/// Determine if a conversion is a complex promotion.
///
/// A complex promotion is defined as a complex -> complex conversion
/// where the conversion between the underlying real types is a
/// floating-point or integral promotion.
bool Sema::IsComplexPromotion(QualType FromType, QualType ToType) {
  const ComplexType *FromComplex = FromType->getAs<ComplexType>();
  if (!FromComplex)
    return false;

  const ComplexType *ToComplex = ToType->getAs<ComplexType>();
  if (!ToComplex)
    return false;

  return IsFloatingPointPromotion(FromComplex->getElementType(),
                                  ToComplex->getElementType()) ||
    IsIntegralPromotion(nullptr, FromComplex->getElementType(),
                        ToComplex->getElementType());
}

/// BuildSimilarlyQualifiedPointerType - In a pointer conversion from
/// the pointer type FromPtr to a pointer to type ToPointee, with the
/// same type qualifiers as FromPtr has on its pointee type. ToType,
/// if non-empty, will be a pointer to ToType that may or may not have
/// the right set of qualifiers on its pointee.
///
static QualType
BuildSimilarlyQualifiedPointerType(const Type *FromPtr,
                                   QualType ToPointee, QualType ToType,
                                   ASTContext &Context,
                                   bool StripObjCLifetime = false) {
  assert((FromPtr->getTypeClass() == Type::Pointer ||
          FromPtr->getTypeClass() == Type::ObjCObjectPointer) &&
         "Invalid similarly-qualified pointer type");

  /// Conversions to 'id' subsume cv-qualifier conversions.
  if (ToType->isObjCIdType() || ToType->isObjCQualifiedIdType())
    return ToType.getUnqualifiedType();

  QualType CanonFromPointee
    = Context.getCanonicalType(FromPtr->getPointeeType());
  QualType CanonToPointee = Context.getCanonicalType(ToPointee);
  Qualifiers Quals = CanonFromPointee.getQualifiers();

  if (StripObjCLifetime)
    Quals.removeObjCLifetime();

  // Exact qualifier match -> return the pointer type we're converting to.
  if (CanonToPointee.getLocalQualifiers() == Quals) {
    // ToType is exactly what we need. Return it.
    if (!ToType.isNull())
      return ToType.getUnqualifiedType();

    // Build a pointer to ToPointee. It has the right qualifiers
    // already.
    if (isa<ObjCObjectPointerType>(ToType))
      return Context.getObjCObjectPointerType(ToPointee);
    return Context.getPointerType(ToPointee);
  }

  // Just build a canonical type that has the right qualifiers.
  QualType QualifiedCanonToPointee
    = Context.getQualifiedType(CanonToPointee.getLocalUnqualifiedType(), Quals);

  if (isa<ObjCObjectPointerType>(ToType))
    return Context.getObjCObjectPointerType(QualifiedCanonToPointee);
  return Context.getPointerType(QualifiedCanonToPointee);
}

static bool isNullPointerConstantForConversion(Expr *Expr,
                                               bool InOverloadResolution,
                                               ASTContext &Context) {
  // Handle value-dependent integral null pointer constants correctly.
  // http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#903
  if (Expr->isValueDependent() && !Expr->isTypeDependent() &&
      Expr->getType()->isIntegerType() && !Expr->getType()->isEnumeralType())
    return !InOverloadResolution;

  return Expr->isNullPointerConstant(Context,
                    InOverloadResolution? Expr::NPC_ValueDependentIsNotNull
                                        : Expr::NPC_ValueDependentIsNull);
}

/// IsPointerConversion - Determines whether the conversion of the
/// expression From, which has the (possibly adjusted) type FromType,
/// can be converted to the type ToType via a pointer conversion (C++
/// 4.10). If so, returns true and places the converted type (that
/// might differ from ToType in its cv-qualifiers at some level) into
/// ConvertedType.
///
/// This routine also supports conversions to and from block pointers
/// and conversions with Objective-C's 'id', 'id<protocols...>', and
/// pointers to interfaces. FIXME: Once we've determined the
/// appropriate overloading rules for Objective-C, we may want to
/// split the Objective-C checks into a different routine; however,
/// GCC seems to consider all of these conversions to be pointer
/// conversions, so for now they live here. IncompatibleObjC will be
/// set if the conversion is an allowed Objective-C conversion that
/// should result in a warning.
bool Sema::IsPointerConversion(Expr *From, QualType FromType, QualType ToType,
                               bool InOverloadResolution,
                               QualType& ConvertedType,
                               bool &IncompatibleObjC) {
  IncompatibleObjC = false;
  if (isObjCPointerConversion(FromType, ToType, ConvertedType,
                              IncompatibleObjC))
    return true;

  // Conversion from a null pointer constant to any Objective-C pointer type.
  if (ToType->isObjCObjectPointerType() &&
      isNullPointerConstantForConversion(From, InOverloadResolution, Context)) {
    ConvertedType = ToType;
    return true;
  }

  // Blocks: Block pointers can be converted to void*.
  if (FromType->isBlockPointerType() && ToType->isPointerType() &&
      ToType->castAs<PointerType>()->getPointeeType()->isVoidType()) {
    ConvertedType = ToType;
    return true;
  }
  // Blocks: A null pointer constant can be converted to a block
  // pointer type.
  if (ToType->isBlockPointerType() &&
      isNullPointerConstantForConversion(From, InOverloadResolution, Context)) {
    ConvertedType = ToType;
    return true;
  }

  // If the left-hand-side is nullptr_t, the right side can be a null
  // pointer constant.
  if (ToType->isNullPtrType() &&
      isNullPointerConstantForConversion(From, InOverloadResolution, Context)) {
    ConvertedType = ToType;
    return true;
  }

  const PointerType* ToTypePtr = ToType->getAs<PointerType>();
  if (!ToTypePtr)
    return false;

  // A null pointer constant can be converted to a pointer type (C++ 4.10p1).
  if (isNullPointerConstantForConversion(From, InOverloadResolution, Context)) {
    ConvertedType = ToType;
    return true;
  }

  // Beyond this point, both types need to be pointers
  // , including objective-c pointers.
  QualType ToPointeeType = ToTypePtr->getPointeeType();
  if (FromType->isObjCObjectPointerType() && ToPointeeType->isVoidType() &&
      !getLangOpts().ObjCAutoRefCount) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(
        FromType->castAs<ObjCObjectPointerType>(), ToPointeeType, ToType,
        Context);
    return true;
  }
  const PointerType *FromTypePtr = FromType->getAs<PointerType>();
  if (!FromTypePtr)
    return false;

  QualType FromPointeeType = FromTypePtr->getPointeeType();

  // If the unqualified pointee types are the same, this can't be a
  // pointer conversion, so don't do all of the work below.
  if (Context.hasSameUnqualifiedType(FromPointeeType, ToPointeeType))
    return false;

  // An rvalue of type "pointer to cv T," where T is an object type,
  // can be converted to an rvalue of type "pointer to cv void" (C++
  // 4.10p2).
  if (FromPointeeType->isIncompleteOrObjectType() &&
      ToPointeeType->isVoidType()) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(FromTypePtr,
                                                       ToPointeeType,
                                                       ToType, Context,
                                                   /*StripObjCLifetime=*/true);
    return true;
  }

  // MSVC allows implicit function to void* type conversion.
  if (getLangOpts().MSVCCompat && FromPointeeType->isFunctionType() &&
      ToPointeeType->isVoidType()) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(FromTypePtr,
                                                       ToPointeeType,
                                                       ToType, Context);
    return true;
  }

  // When we're overloading in C, we allow a special kind of pointer
  // conversion for compatible-but-not-identical pointee types.
  if (!getLangOpts().CPlusPlus &&
      Context.typesAreCompatible(FromPointeeType, ToPointeeType)) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(FromTypePtr,
                                                       ToPointeeType,
                                                       ToType, Context);
    return true;
  }

  // C++ [conv.ptr]p3:
  //
  //   An rvalue of type "pointer to cv D," where D is a class type,
  //   can be converted to an rvalue of type "pointer to cv B," where
  //   B is a base class (clause 10) of D. If B is an inaccessible
  //   (clause 11) or ambiguous (10.2) base class of D, a program that
  //   necessitates this conversion is ill-formed. The result of the
  //   conversion is a pointer to the base class sub-object of the
  //   derived class object. The null pointer value is converted to
  //   the null pointer value of the destination type.
  //
  // Note that we do not check for ambiguity or inaccessibility
  // here. That is handled by CheckPointerConversion.
  if (getLangOpts().CPlusPlus && FromPointeeType->isRecordType() &&
      ToPointeeType->isRecordType() &&
      !Context.hasSameUnqualifiedType(FromPointeeType, ToPointeeType) &&
      IsDerivedFrom(From->getBeginLoc(), FromPointeeType, ToPointeeType)) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(FromTypePtr,
                                                       ToPointeeType,
                                                       ToType, Context);
    return true;
  }

  if (FromPointeeType->isVectorType() && ToPointeeType->isVectorType() &&
      Context.areCompatibleVectorTypes(FromPointeeType, ToPointeeType)) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(FromTypePtr,
                                                       ToPointeeType,
                                                       ToType, Context);
    return true;
  }

  return false;
}

/// Adopt the given qualifiers for the given type.
static QualType AdoptQualifiers(ASTContext &Context, QualType T, Qualifiers Qs){
  Qualifiers TQs = T.getQualifiers();

  // Check whether qualifiers already match.
  if (TQs == Qs)
    return T;

  if (Qs.compatiblyIncludes(TQs))
    return Context.getQualifiedType(T, Qs);

  return Context.getQualifiedType(T.getUnqualifiedType(), Qs);
}

/// isObjCPointerConversion - Determines whether this is an
/// Objective-C pointer conversion. Subroutine of IsPointerConversion,
/// with the same arguments and return values.
bool Sema::isObjCPointerConversion(QualType FromType, QualType ToType,
                                   QualType& ConvertedType,
                                   bool &IncompatibleObjC) {
  if (!getLangOpts().ObjC)
    return false;

  // The set of qualifiers on the type we're converting from.
  Qualifiers FromQualifiers = FromType.getQualifiers();

  // First, we handle all conversions on ObjC object pointer types.
  const ObjCObjectPointerType* ToObjCPtr =
    ToType->getAs<ObjCObjectPointerType>();
  const ObjCObjectPointerType *FromObjCPtr =
    FromType->getAs<ObjCObjectPointerType>();

  if (ToObjCPtr && FromObjCPtr) {
    // If the pointee types are the same (ignoring qualifications),
    // then this is not a pointer conversion.
    if (Context.hasSameUnqualifiedType(ToObjCPtr->getPointeeType(),
                                       FromObjCPtr->getPointeeType()))
      return false;

    // Conversion between Objective-C pointers.
    if (Context.canAssignObjCInterfaces(ToObjCPtr, FromObjCPtr)) {
      const ObjCInterfaceType* LHS = ToObjCPtr->getInterfaceType();
      const ObjCInterfaceType* RHS = FromObjCPtr->getInterfaceType();
      if (getLangOpts().CPlusPlus && LHS && RHS &&
          !ToObjCPtr->getPointeeType().isAtLeastAsQualifiedAs(
                                                FromObjCPtr->getPointeeType()))
        return false;
      ConvertedType = BuildSimilarlyQualifiedPointerType(FromObjCPtr,
                                                   ToObjCPtr->getPointeeType(),
                                                         ToType, Context);
      ConvertedType = AdoptQualifiers(Context, ConvertedType, FromQualifiers);
      return true;
    }

    if (Context.canAssignObjCInterfaces(FromObjCPtr, ToObjCPtr)) {
      // Okay: this is some kind of implicit downcast of Objective-C
      // interfaces, which is permitted. However, we're going to
      // complain about it.
      IncompatibleObjC = true;
      ConvertedType = BuildSimilarlyQualifiedPointerType(FromObjCPtr,
                                                   ToObjCPtr->getPointeeType(),
                                                         ToType, Context);
      ConvertedType = AdoptQualifiers(Context, ConvertedType, FromQualifiers);
      return true;
    }
  }
  // Beyond this point, both types need to be C pointers or block pointers.
  QualType ToPointeeType;
  if (const PointerType *ToCPtr = ToType->getAs<PointerType>())
    ToPointeeType = ToCPtr->getPointeeType();
  else if (const BlockPointerType *ToBlockPtr =
            ToType->getAs<BlockPointerType>()) {
    // Objective C++: We're able to convert from a pointer to any object
    // to a block pointer type.
    if (FromObjCPtr && FromObjCPtr->isObjCBuiltinType()) {
      ConvertedType = AdoptQualifiers(Context, ToType, FromQualifiers);
      return true;
    }
    ToPointeeType = ToBlockPtr->getPointeeType();
  }
  else if (FromType->getAs<BlockPointerType>() &&
           ToObjCPtr && ToObjCPtr->isObjCBuiltinType()) {
    // Objective C++: We're able to convert from a block pointer type to a
    // pointer to any object.
    ConvertedType = AdoptQualifiers(Context, ToType, FromQualifiers);
    return true;
  }
  else
    return false;

  QualType FromPointeeType;
  if (const PointerType *FromCPtr = FromType->getAs<PointerType>())
    FromPointeeType = FromCPtr->getPointeeType();
  else if (const BlockPointerType *FromBlockPtr =
           FromType->getAs<BlockPointerType>())
    FromPointeeType = FromBlockPtr->getPointeeType();
  else
    return false;

  // If we have pointers to pointers, recursively check whether this
  // is an Objective-C conversion.
  if (FromPointeeType->isPointerType() && ToPointeeType->isPointerType() &&
      isObjCPointerConversion(FromPointeeType, ToPointeeType, ConvertedType,
                              IncompatibleObjC)) {
    // We always complain about this conversion.
    IncompatibleObjC = true;
    ConvertedType = Context.getPointerType(ConvertedType);
    ConvertedType = AdoptQualifiers(Context, ConvertedType, FromQualifiers);
    return true;
  }
  // Allow conversion of pointee being objective-c pointer to another one;
  // as in I* to id.
  if (FromPointeeType->getAs<ObjCObjectPointerType>() &&
      ToPointeeType->getAs<ObjCObjectPointerType>() &&
      isObjCPointerConversion(FromPointeeType, ToPointeeType, ConvertedType,
                              IncompatibleObjC)) {

    ConvertedType = Context.getPointerType(ConvertedType);
    ConvertedType = AdoptQualifiers(Context, ConvertedType, FromQualifiers);
    return true;
  }

  // If we have pointers to functions or blocks, check whether the only
  // differences in the argument and result types are in Objective-C
  // pointer conversions. If so, we permit the conversion (but
  // complain about it).
  const FunctionProtoType *FromFunctionType
    = FromPointeeType->getAs<FunctionProtoType>();
  const FunctionProtoType *ToFunctionType
    = ToPointeeType->getAs<FunctionProtoType>();
  if (FromFunctionType && ToFunctionType) {
    // If the function types are exactly the same, this isn't an
    // Objective-C pointer conversion.
    if (Context.getCanonicalType(FromPointeeType)
          == Context.getCanonicalType(ToPointeeType))
      return false;

    // Perform the quick checks that will tell us whether these
    // function types are obviously different.
    if (FromFunctionType->getNumParams() != ToFunctionType->getNumParams() ||
        FromFunctionType->isVariadic() != ToFunctionType->isVariadic() ||
        FromFunctionType->getMethodQuals() != ToFunctionType->getMethodQuals())
      return false;

    bool HasObjCConversion = false;
    if (Context.getCanonicalType(FromFunctionType->getReturnType()) ==
        Context.getCanonicalType(ToFunctionType->getReturnType())) {
      // Okay, the types match exactly. Nothing to do.
    } else if (isObjCPointerConversion(FromFunctionType->getReturnType(),
                                       ToFunctionType->getReturnType(),
                                       ConvertedType, IncompatibleObjC)) {
      // Okay, we have an Objective-C pointer conversion.
      HasObjCConversion = true;
    } else {
      // Function types are too different. Abort.
      return false;
    }

    // Check argument types.
    for (unsigned ArgIdx = 0, NumArgs = FromFunctionType->getNumParams();
         ArgIdx != NumArgs; ++ArgIdx) {
      QualType FromArgType = FromFunctionType->getParamType(ArgIdx);
      QualType ToArgType = ToFunctionType->getParamType(ArgIdx);
      if (Context.getCanonicalType(FromArgType)
            == Context.getCanonicalType(ToArgType)) {
        // Okay, the types match exactly. Nothing to do.
      } else if (isObjCPointerConversion(FromArgType, ToArgType,
                                         ConvertedType, IncompatibleObjC)) {
        // Okay, we have an Objective-C pointer conversion.
        HasObjCConversion = true;
      } else {
        // Argument types are too different. Abort.
        return false;
      }
    }

    if (HasObjCConversion) {
      // We had an Objective-C conversion. Allow this pointer
      // conversion, but complain about it.
      ConvertedType = AdoptQualifiers(Context, ToType, FromQualifiers);
      IncompatibleObjC = true;
      return true;
    }
  }

  return false;
}

/// Determine whether this is an Objective-C writeback conversion,
/// used for parameter passing when performing automatic reference counting.
///
/// \param FromType The type we're converting form.
///
/// \param ToType The type we're converting to.
///
/// \param ConvertedType The type that will be produced after applying
/// this conversion.
bool Sema::isObjCWritebackConversion(QualType FromType, QualType ToType,
                                     QualType &ConvertedType) {
  if (!getLangOpts().ObjCAutoRefCount ||
      Context.hasSameUnqualifiedType(FromType, ToType))
    return false;

  // Parameter must be a pointer to __autoreleasing (with no other qualifiers).
  QualType ToPointee;
  if (const PointerType *ToPointer = ToType->getAs<PointerType>())
    ToPointee = ToPointer->getPointeeType();
  else
    return false;

  Qualifiers ToQuals = ToPointee.getQualifiers();
  if (!ToPointee->isObjCLifetimeType() ||
      ToQuals.getObjCLifetime() != Qualifiers::OCL_Autoreleasing ||
      !ToQuals.withoutObjCLifetime().empty())
    return false;

  // Argument must be a pointer to __strong to __weak.
  QualType FromPointee;
  if (const PointerType *FromPointer = FromType->getAs<PointerType>())
    FromPointee = FromPointer->getPointeeType();
  else
    return false;

  Qualifiers FromQuals = FromPointee.getQualifiers();
  if (!FromPointee->isObjCLifetimeType() ||
      (FromQuals.getObjCLifetime() != Qualifiers::OCL_Strong &&
       FromQuals.getObjCLifetime() != Qualifiers::OCL_Weak))
    return false;

  // Make sure that we have compatible qualifiers.
  FromQuals.setObjCLifetime(Qualifiers::OCL_Autoreleasing);
  if (!ToQuals.compatiblyIncludes(FromQuals))
    return false;

  // Remove qualifiers from the pointee type we're converting from; they
  // aren't used in the compatibility check belong, and we'll be adding back
  // qualifiers (with __autoreleasing) if the compatibility check succeeds.
  FromPointee = FromPointee.getUnqualifiedType();

  // The unqualified form of the pointee types must be compatible.
  ToPointee = ToPointee.getUnqualifiedType();
  bool IncompatibleObjC;
  if (Context.typesAreCompatible(FromPointee, ToPointee))
    FromPointee = ToPointee;
  else if (!isObjCPointerConversion(FromPointee, ToPointee, FromPointee,
                                    IncompatibleObjC))
    return false;

  /// Construct the type we're converting to, which is a pointer to
  /// __autoreleasing pointee.
  FromPointee = Context.getQualifiedType(FromPointee, FromQuals);
  ConvertedType = Context.getPointerType(FromPointee);
  return true;
}

bool Sema::IsBlockPointerConversion(QualType FromType, QualType ToType,
                                    QualType& ConvertedType) {
  QualType ToPointeeType;
  if (const BlockPointerType *ToBlockPtr =
        ToType->getAs<BlockPointerType>())
    ToPointeeType = ToBlockPtr->getPointeeType();
  else
    return false;

  QualType FromPointeeType;
  if (const BlockPointerType *FromBlockPtr =
      FromType->getAs<BlockPointerType>())
    FromPointeeType = FromBlockPtr->getPointeeType();
  else
    return false;
  // We have pointer to blocks, check whether the only
  // differences in the argument and result types are in Objective-C
  // pointer conversions. If so, we permit the conversion.

  const FunctionProtoType *FromFunctionType
    = FromPointeeType->getAs<FunctionProtoType>();
  const FunctionProtoType *ToFunctionType
    = ToPointeeType->getAs<FunctionProtoType>();

  if (!FromFunctionType || !ToFunctionType)
    return false;

  if (Context.hasSameType(FromPointeeType, ToPointeeType))
    return true;

  // Perform the quick checks that will tell us whether these
  // function types are obviously different.
  if (FromFunctionType->getNumParams() != ToFunctionType->getNumParams() ||
      FromFunctionType->isVariadic() != ToFunctionType->isVariadic())
    return false;

  FunctionType::ExtInfo FromEInfo = FromFunctionType->getExtInfo();
  FunctionType::ExtInfo ToEInfo = ToFunctionType->getExtInfo();
  if (FromEInfo != ToEInfo)
    return false;

  bool IncompatibleObjC = false;
  if (Context.hasSameType(FromFunctionType->getReturnType(),
                          ToFunctionType->getReturnType())) {
    // Okay, the types match exactly. Nothing to do.
  } else {
    QualType RHS = FromFunctionType->getReturnType();
    QualType LHS = ToFunctionType->getReturnType();
    if ((!getLangOpts().CPlusPlus || !RHS->isRecordType()) &&
        !RHS.hasQualifiers() && LHS.hasQualifiers())
       LHS = LHS.getUnqualifiedType();

     if (Context.hasSameType(RHS,LHS)) {
       // OK exact match.
     } else if (isObjCPointerConversion(RHS, LHS,
                                        ConvertedType, IncompatibleObjC)) {
     if (IncompatibleObjC)
       return false;
     // Okay, we have an Objective-C pointer conversion.
     }
     else
       return false;
   }

   // Check argument types.
   for (unsigned ArgIdx = 0, NumArgs = FromFunctionType->getNumParams();
        ArgIdx != NumArgs; ++ArgIdx) {
     IncompatibleObjC = false;
     QualType FromArgType = FromFunctionType->getParamType(ArgIdx);
     QualType ToArgType = ToFunctionType->getParamType(ArgIdx);
     if (Context.hasSameType(FromArgType, ToArgType)) {
       // Okay, the types match exactly. Nothing to do.
     } else if (isObjCPointerConversion(ToArgType, FromArgType,
                                        ConvertedType, IncompatibleObjC)) {
       if (IncompatibleObjC)
         return false;
       // Okay, we have an Objective-C pointer conversion.
     } else
       // Argument types are too different. Abort.
       return false;
   }

   SmallVector<FunctionProtoType::ExtParameterInfo, 4> NewParamInfos;
   bool CanUseToFPT, CanUseFromFPT;
   if (!Context.mergeExtParameterInfo(ToFunctionType, FromFunctionType,
                                      CanUseToFPT, CanUseFromFPT,
                                      NewParamInfos))
     return false;

   ConvertedType = ToType;
   return true;
}

enum {
  ft_default,
  ft_different_class,
  ft_parameter_arity,
  ft_parameter_mismatch,
  ft_return_type,
  ft_qualifer_mismatch,
  ft_noexcept
};

/// Attempts to get the FunctionProtoType from a Type. Handles
/// MemberFunctionPointers properly.
static const FunctionProtoType *tryGetFunctionProtoType(QualType FromType) {
  if (auto *FPT = FromType->getAs<FunctionProtoType>())
    return FPT;

  if (auto *MPT = FromType->getAs<MemberPointerType>())
    return MPT->getPointeeType()->getAs<FunctionProtoType>();

  return nullptr;
}

/// HandleFunctionTypeMismatch - Gives diagnostic information for differeing
/// function types.  Catches different number of parameter, mismatch in
/// parameter types, and different return types.
void Sema::HandleFunctionTypeMismatch(PartialDiagnostic &PDiag,
                                      QualType FromType, QualType ToType) {
  // If either type is not valid, include no extra info.
  if (FromType.isNull() || ToType.isNull()) {
    PDiag << ft_default;
    return;
  }

  // Get the function type from the pointers.
  if (FromType->isMemberPointerType() && ToType->isMemberPointerType()) {
    const auto *FromMember = FromType->castAs<MemberPointerType>(),
               *ToMember = ToType->castAs<MemberPointerType>();
    if (!Context.hasSameType(FromMember->getClass(), ToMember->getClass())) {
      PDiag << ft_different_class << QualType(ToMember->getClass(), 0)
            << QualType(FromMember->getClass(), 0);
      return;
    }
    FromType = FromMember->getPointeeType();
    ToType = ToMember->getPointeeType();
  }

  if (FromType->isPointerType())
    FromType = FromType->getPointeeType();
  if (ToType->isPointerType())
    ToType = ToType->getPointeeType();

  // Remove references.
  FromType = FromType.getNonReferenceType();
  ToType = ToType.getNonReferenceType();

  // Don't print extra info for non-specialized template functions.
  if (FromType->isInstantiationDependentType() &&
      !FromType->getAs<TemplateSpecializationType>()) {
    PDiag << ft_default;
    return;
  }

  // No extra info for same types.
  if (Context.hasSameType(FromType, ToType)) {
    PDiag << ft_default;
    return;
  }

  const FunctionProtoType *FromFunction = tryGetFunctionProtoType(FromType),
                          *ToFunction = tryGetFunctionProtoType(ToType);

  // Both types need to be function types.
  if (!FromFunction || !ToFunction) {
    PDiag << ft_default;
    return;
  }

  if (FromFunction->getNumParams() != ToFunction->getNumParams()) {
    PDiag << ft_parameter_arity << ToFunction->getNumParams()
          << FromFunction->getNumParams();
    return;
  }

  // Handle different parameter types.
  unsigned ArgPos;
  if (!FunctionParamTypesAreEqual(FromFunction, ToFunction, &ArgPos)) {
    PDiag << ft_parameter_mismatch << ArgPos + 1
          << ToFunction->getParamType(ArgPos)
          << FromFunction->getParamType(ArgPos);
    return;
  }

  // Handle different return type.
  if (!Context.hasSameType(FromFunction->getReturnType(),
                           ToFunction->getReturnType())) {
    PDiag << ft_return_type << ToFunction->getReturnType()
          << FromFunction->getReturnType();
    return;
  }

  if (FromFunction->getMethodQuals() != ToFunction->getMethodQuals()) {
    PDiag << ft_qualifer_mismatch << ToFunction->getMethodQuals()
          << FromFunction->getMethodQuals();
    return;
  }

  // Handle exception specification differences on canonical type (in C++17
  // onwards).
  if (cast<FunctionProtoType>(FromFunction->getCanonicalTypeUnqualified())
          ->isNothrow() !=
      cast<FunctionProtoType>(ToFunction->getCanonicalTypeUnqualified())
          ->isNothrow()) {
    PDiag << ft_noexcept;
    return;
  }

  // Unable to find a difference, so add no extra info.
  PDiag << ft_default;
}

/// FunctionParamTypesAreEqual - This routine checks two function proto types
/// for equality of their parameter types. Caller has already checked that
/// they have same number of parameters.  If the parameters are different,
/// ArgPos will have the parameter index of the first different parameter.
/// If `Reversed` is true, the parameters of `NewType` will be compared in
/// reverse order. That's useful if one of the functions is being used as a C++20
/// synthesized operator overload with a reversed parameter order.
bool Sema::FunctionParamTypesAreEqual(const FunctionProtoType *OldType,
                                      const FunctionProtoType *NewType,
                                      unsigned *ArgPos, bool Reversed) {
  assert(OldType->getNumParams() == NewType->getNumParams() &&
         "Can't compare parameters of functions with different number of "
         "parameters!");
  for (size_t I = 0; I < OldType->getNumParams(); I++) {
    // Reverse iterate over the parameters of `OldType` if `Reversed` is true.
    size_t J = Reversed ? (OldType->getNumParams() - I - 1) : I;

    // Ignore address spaces in pointee type. This is to disallow overloading
    // on __ptr32/__ptr64 address spaces.
    QualType Old = Context.removePtrSizeAddrSpace(OldType->getParamType(I).getUnqualifiedType());
    QualType New = Context.removePtrSizeAddrSpace(NewType->getParamType(J).getUnqualifiedType());

    if (!Context.hasSameType(Old, New)) {
      if (ArgPos)
        *ArgPos = I;
      return false;
    }
  }
  return true;
}

/// CheckPointerConversion - Check the pointer conversion from the
/// expression From to the type ToType. This routine checks for
/// ambiguous or inaccessible derived-to-base pointer
/// conversions for which IsPointerConversion has already returned
/// true. It returns true and produces a diagnostic if there was an
/// error, or returns false otherwise.
bool Sema::CheckPointerConversion(Expr *From, QualType ToType,
                                  CastKind &Kind,
                                  CXXCastPath& BasePath,
                                  bool IgnoreBaseAccess,
                                  bool Diagnose) {
  QualType FromType = From->getType();
  bool IsCStyleOrFunctionalCast = IgnoreBaseAccess;

  Kind = CK_BitCast;

  if (Diagnose && !IsCStyleOrFunctionalCast && !FromType->isAnyPointerType() &&
      From->isNullPointerConstant(Context, Expr::NPC_ValueDependentIsNotNull) ==
          Expr::NPCK_ZeroExpression) {
    if (Context.hasSameUnqualifiedType(From->getType(), Context.BoolTy))
      DiagRuntimeBehavior(From->getExprLoc(), From,
                          PDiag(diag::warn_impcast_bool_to_null_pointer)
                            << ToType << From->getSourceRange());
    else if (!isUnevaluatedContext())
      Diag(From->getExprLoc(), diag::warn_non_literal_null_pointer)
        << ToType << From->getSourceRange();
  }
  if (const PointerType *ToPtrType = ToType->getAs<PointerType>()) {
    if (const PointerType *FromPtrType = FromType->getAs<PointerType>()) {
      QualType FromPointeeType = FromPtrType->getPointeeType(),
               ToPointeeType   = ToPtrType->getPointeeType();

      if (FromPointeeType->isRecordType() && ToPointeeType->isRecordType() &&
          !Context.hasSameUnqualifiedType(FromPointeeType, ToPointeeType)) {
        // We must have a derived-to-base conversion. Check an
        // ambiguous or inaccessible conversion.
        unsigned InaccessibleID = 0;
        unsigned AmbiguousID = 0;
        if (Diagnose) {
          InaccessibleID = diag::err_upcast_to_inaccessible_base;
          AmbiguousID = diag::err_ambiguous_derived_to_base_conv;
        }
        if (CheckDerivedToBaseConversion(
                FromPointeeType, ToPointeeType, InaccessibleID, AmbiguousID,
                From->getExprLoc(), From->getSourceRange(), DeclarationName(),
                &BasePath, IgnoreBaseAccess))
          return true;

        // The conversion was successful.
        Kind = CK_DerivedToBase;
      }

      if (Diagnose && !IsCStyleOrFunctionalCast &&
          FromPointeeType->isFunctionType() && ToPointeeType->isVoidType()) {
        assert(getLangOpts().MSVCCompat &&
               "this should only be possible with MSVCCompat!");
        Diag(From->getExprLoc(), diag::ext_ms_impcast_fn_obj)
            << From->getSourceRange();
      }
    }
  } else if (const ObjCObjectPointerType *ToPtrType =
               ToType->getAs<ObjCObjectPointerType>()) {
    if (const ObjCObjectPointerType *FromPtrType =
          FromType->getAs<ObjCObjectPointerType>()) {
      // Objective-C++ conversions are always okay.
      // FIXME: We should have a different class of conversions for the
      // Objective-C++ implicit conversions.
      if (FromPtrType->isObjCBuiltinType() || ToPtrType->isObjCBuiltinType())
        return false;
    } else if (FromType->isBlockPointerType()) {
      Kind = CK_BlockPointerToObjCPointerCast;
    } else {
      Kind = CK_CPointerToObjCPointerCast;
    }
  } else if (ToType->isBlockPointerType()) {
    if (!FromType->isBlockPointerType())
      Kind = CK_AnyPointerToBlockPointerCast;
  }

  // We shouldn't fall into this case unless it's valid for other
  // reasons.
  if (From->isNullPointerConstant(Context, Expr::NPC_ValueDependentIsNull))
    Kind = CK_NullToPointer;

  return false;
}

/// IsMemberPointerConversion - Determines whether the conversion of the
/// expression From, which has the (possibly adjusted) type FromType, can be
/// converted to the type ToType via a member pointer conversion (C++ 4.11).
/// If so, returns true and places the converted type (that might differ from
/// ToType in its cv-qualifiers at some level) into ConvertedType.
bool Sema::IsMemberPointerConversion(Expr *From, QualType FromType,
                                     QualType ToType,
                                     bool InOverloadResolution,
                                     QualType &ConvertedType) {
  const MemberPointerType *ToTypePtr = ToType->getAs<MemberPointerType>();
  if (!ToTypePtr)
    return false;

  // A null pointer constant can be converted to a member pointer (C++ 4.11p1)
  if (From->isNullPointerConstant(Context,
                    InOverloadResolution? Expr::NPC_ValueDependentIsNotNull
                                        : Expr::NPC_ValueDependentIsNull)) {
    ConvertedType = ToType;
    return true;
  }

  // Otherwise, both types have to be member pointers.
  const MemberPointerType *FromTypePtr = FromType->getAs<MemberPointerType>();
  if (!FromTypePtr)
    return false;

  // A pointer to member of B can be converted to a pointer to member of D,
  // where D is derived from B (C++ 4.11p2).
  QualType FromClass(FromTypePtr->getClass(), 0);
  QualType ToClass(ToTypePtr->getClass(), 0);

  if (!Context.hasSameUnqualifiedType(FromClass, ToClass) &&
      IsDerivedFrom(From->getBeginLoc(), ToClass, FromClass)) {
    ConvertedType = Context.getMemberPointerType(FromTypePtr->getPointeeType(),
                                                 ToClass.getTypePtr());
    return true;
  }

  return false;
}

/// CheckMemberPointerConversion - Check the member pointer conversion from the
/// expression From to the type ToType. This routine checks for ambiguous or
/// virtual or inaccessible base-to-derived member pointer conversions
/// for which IsMemberPointerConversion has already returned true. It returns
/// true and produces a diagnostic if there was an error, or returns false
/// otherwise.
bool Sema::CheckMemberPointerConversion(Expr *From, QualType ToType,
                                        CastKind &Kind,
                                        CXXCastPath &BasePath,
                                        bool IgnoreBaseAccess) {
  QualType FromType = From->getType();
  const MemberPointerType *FromPtrType = FromType->getAs<MemberPointerType>();
  if (!FromPtrType) {
    // This must be a null pointer to member pointer conversion
    assert(From->isNullPointerConstant(Context,
                                       Expr::NPC_ValueDependentIsNull) &&
           "Expr must be null pointer constant!");
    Kind = CK_NullToMemberPointer;
    return false;
  }

  const MemberPointerType *ToPtrType = ToType->getAs<MemberPointerType>();
  assert(ToPtrType && "No member pointer cast has a target type "
                      "that is not a member pointer.");

  QualType FromClass = QualType(FromPtrType->getClass(), 0);
  QualType ToClass   = QualType(ToPtrType->getClass(), 0);

  // FIXME: What about dependent types?
  assert(FromClass->isRecordType() && "Pointer into non-class.");
  assert(ToClass->isRecordType() && "Pointer into non-class.");

  CXXBasePaths Paths(/*FindAmbiguities=*/true, /*RecordPaths=*/true,
                     /*DetectVirtual=*/true);
  bool DerivationOkay =
      IsDerivedFrom(From->getBeginLoc(), ToClass, FromClass, Paths);
  assert(DerivationOkay &&
         "Should not have been called if derivation isn't OK.");
  (void)DerivationOkay;

  if (Paths.isAmbiguous(Context.getCanonicalType(FromClass).
                                  getUnqualifiedType())) {
    std::string PathDisplayStr = getAmbiguousPathsDisplayString(Paths);
    Diag(From->getExprLoc(), diag::err_ambiguous_memptr_conv)
      << 0 << FromClass << ToClass << PathDisplayStr << From->getSourceRange();
    return true;
  }

  if (const RecordType *VBase = Paths.getDetectedVirtual()) {
    Diag(From->getExprLoc(), diag::err_memptr_conv_via_virtual)
      << FromClass << ToClass << QualType(VBase, 0)
      << From->getSourceRange();
    return true;
  }

  if (!IgnoreBaseAccess)
    CheckBaseClassAccess(From->getExprLoc(), FromClass, ToClass,
                         Paths.front(),
                         diag::err_downcast_from_inaccessible_base);

  // Must be a base to derived member conversion.
  BuildBasePathArray(Paths, BasePath);
  Kind = CK_BaseToDerivedMemberPointer;
  return false;
}

/// Determine whether the lifetime conversion between the two given
/// qualifiers sets is nontrivial.
static bool isNonTrivialObjCLifetimeConversion(Qualifiers FromQuals,
                                               Qualifiers ToQuals) {
  // Converting anything to const __unsafe_unretained is trivial.
  if (ToQuals.hasConst() &&
      ToQuals.getObjCLifetime() == Qualifiers::OCL_ExplicitNone)
    return false;

  return true;
}

/// Perform a single iteration of the loop for checking if a qualification
/// conversion is valid.
///
/// Specifically, check whether any change between the qualifiers of \p
/// FromType and \p ToType is permissible, given knowledge about whether every
/// outer layer is const-qualified.
static bool isQualificationConversionStep(QualType FromType, QualType ToType,
                                          bool CStyle, bool IsTopLevel,
                                          bool &PreviousToQualsIncludeConst,
                                          bool &ObjCLifetimeConversion) {
  Qualifiers FromQuals = FromType.getQualifiers();
  Qualifiers ToQuals = ToType.getQualifiers();

  // Ignore __unaligned qualifier.
  FromQuals.removeUnaligned();

  // Objective-C ARC:
  //   Check Objective-C lifetime conversions.
  if (FromQuals.getObjCLifetime() != ToQuals.getObjCLifetime()) {
    if (ToQuals.compatiblyIncludesObjCLifetime(FromQuals)) {
      if (isNonTrivialObjCLifetimeConversion(FromQuals, ToQuals))
        ObjCLifetimeConversion = true;
      FromQuals.removeObjCLifetime();
      ToQuals.removeObjCLifetime();
    } else {
      // Qualification conversions cannot cast between different
      // Objective-C lifetime qualifiers.
      return false;
    }
  }

  // Allow addition/removal of GC attributes but not changing GC attributes.
  if (FromQuals.getObjCGCAttr() != ToQuals.getObjCGCAttr() &&
      (!FromQuals.hasObjCGCAttr() || !ToQuals.hasObjCGCAttr())) {
    FromQuals.removeObjCGCAttr();
    ToQuals.removeObjCGCAttr();
  }

  //   -- for every j > 0, if const is in cv 1,j then const is in cv
  //      2,j, and similarly for volatile.
  if (!CStyle && !ToQuals.compatiblyIncludes(FromQuals))
    return false;

  // If address spaces mismatch:
  //  - in top level it is only valid to convert to addr space that is a
  //    superset in all cases apart from C-style casts where we allow
  //    conversions between overlapping address spaces.
  //  - in non-top levels it is not a valid conversion.
  if (ToQuals.getAddressSpace() != FromQuals.getAddressSpace() &&
      (!IsTopLevel ||
       !(ToQuals.isAddressSpaceSupersetOf(FromQuals) ||
         (CStyle && FromQuals.isAddressSpaceSupersetOf(ToQuals)))))
    return false;

  //   -- if the cv 1,j and cv 2,j are different, then const is in
  //      every cv for 0 < k < j.
  if (!CStyle && FromQuals.getCVRQualifiers() != ToQuals.getCVRQualifiers() &&
      !PreviousToQualsIncludeConst)
    return false;

  // The following wording is from C++20, where the result of the conversion
  // is T3, not T2.
  //   -- if [...] P1,i [...] is "array of unknown bound of", P3,i is
  //      "array of unknown bound of"
  if (FromType->isIncompleteArrayType() && !ToType->isIncompleteArrayType())
    return false;

  //   -- if the resulting P3,i is different from P1,i [...], then const is
  //      added to every cv 3_k for 0 < k < i.
  if (!CStyle && FromType->isConstantArrayType() &&
      ToType->isIncompleteArrayType() && !PreviousToQualsIncludeConst)
    return false;

  // Keep track of whether all prior cv-qualifiers in the "to" type
  // include const.
  PreviousToQualsIncludeConst =
      PreviousToQualsIncludeConst && ToQuals.hasConst();
  return true;
}

/// IsQualificationConversion - Determines whether the conversion from
/// an rvalue of type FromType to ToType is a qualification conversion
/// (C++ 4.4).
///
/// \param ObjCLifetimeConversion Output parameter that will be set to indicate
/// when the qualification conversion involves a change in the Objective-C
/// object lifetime.
bool
Sema::IsQualificationConversion(QualType FromType, QualType ToType,
                                bool CStyle, bool &ObjCLifetimeConversion) {
  FromType = Context.getCanonicalType(FromType);
  ToType = Context.getCanonicalType(ToType);
  ObjCLifetimeConversion = false;

  // If FromType and ToType are the same type, this is not a
  // qualification conversion.
  if (FromType.getUnqualifiedType() == ToType.getUnqualifiedType())
    return false;

  // (C++ 4.4p4):
  //   A conversion can add cv-qualifiers at levels other than the first
  //   in multi-level pointers, subject to the following rules: [...]
  bool PreviousToQualsIncludeConst = true;
  bool UnwrappedAnyPointer = false;
  while (Context.UnwrapSimilarTypes(FromType, ToType)) {
    if (!isQualificationConversionStep(
            FromType, ToType, CStyle, !UnwrappedAnyPointer,
            PreviousToQualsIncludeConst, ObjCLifetimeConversion))
      return false;
    UnwrappedAnyPointer = true;
  }

  // We are left with FromType and ToType being the pointee types
  // after unwrapping the original FromType and ToType the same number
  // of times. If we unwrapped any pointers, and if FromType and
  // ToType have the same unqualified type (since we checked
  // qualifiers above), then this is a qualification conversion.
  return UnwrappedAnyPointer && Context.hasSameUnqualifiedType(FromType,ToType);
}

/// - Determine whether this is a conversion from a scalar type to an
/// atomic type.
///
/// If successful, updates \c SCS's second and third steps in the conversion
/// sequence to finish the conversion.
static bool tryAtomicConversion(Sema &S, Expr *From, QualType ToType,
                                bool InOverloadResolution,
                                StandardConversionSequence &SCS,
                                bool CStyle) {
  const AtomicType *ToAtomic = ToType->getAs<AtomicType>();
  if (!ToAtomic)
    return false;

  StandardConversionSequence InnerSCS;
  if (!IsStandardConversion(S, From, ToAtomic->getValueType(),
                            InOverloadResolution, InnerSCS,
                            CStyle, /*AllowObjCWritebackConversion=*/false))
    return false;

  SCS.Second = InnerSCS.Second;
  SCS.setToType(1, InnerSCS.getToType(1));
  SCS.Third = InnerSCS.Third;
  SCS.QualificationIncludesObjCLifetime
    = InnerSCS.QualificationIncludesObjCLifetime;
  SCS.setToType(2, InnerSCS.getToType(2));
  return true;
}

static bool isFirstArgumentCompatibleWithType(ASTContext &Context,
                                              CXXConstructorDecl *Constructor,
                                              QualType Type) {
  const auto *CtorType = Constructor->getType()->castAs<FunctionProtoType>();
  if (CtorType->getNumParams() > 0) {
    QualType FirstArg = CtorType->getParamType(0);
    if (Context.hasSameUnqualifiedType(Type, FirstArg.getNonReferenceType()))
      return true;
  }
  return false;
}

static OverloadingResult
IsInitializerListConstructorConversion(Sema &S, Expr *From, QualType ToType,
                                       CXXRecordDecl *To,
                                       UserDefinedConversionSequence &User,
                                       OverloadCandidateSet &CandidateSet,
                                       bool AllowExplicit) {
  CandidateSet.clear(OverloadCandidateSet::CSK_InitByUserDefinedConversion);
  for (auto *D : S.LookupConstructors(To)) {
    auto Info = getConstructorInfo(D);
    if (!Info)
      continue;

    bool Usable = !Info.Constructor->isInvalidDecl() &&
                  S.isInitListConstructor(Info.Constructor);
    if (Usable) {
      bool SuppressUserConversions = false;
      if (Info.ConstructorTmpl)
        S.AddTemplateOverloadCandidate(Info.ConstructorTmpl, Info.FoundDecl,
                                       /*ExplicitArgs*/ nullptr, From,
                                       CandidateSet, SuppressUserConversions,
                                       /*PartialOverloading*/ false,
                                       AllowExplicit);
      else
        S.AddOverloadCandidate(Info.Constructor, Info.FoundDecl, From,
                               CandidateSet, SuppressUserConversions,
                               /*PartialOverloading*/ false, AllowExplicit);
    }
  }

  bool HadMultipleCandidates = (CandidateSet.size() > 1);

  OverloadCandidateSet::iterator Best;
  switch (auto Result =
              CandidateSet.BestViableFunction(S, From->getBeginLoc(), Best)) {
  case OR_Deleted:
  case OR_Success: {
    // Record the standard conversion we used and the conversion function.
    CXXConstructorDecl *Constructor = cast<CXXConstructorDecl>(Best->Function);
    QualType ThisType = Constructor->getThisType();
    // Initializer lists don't have conversions as such.
    User.Before.setAsIdentityConversion();
    User.HadMultipleCandidates = HadMultipleCandidates;
    User.ConversionFunction = Constructor;
    User.FoundConversionFunction = Best->FoundDecl;
    User.After.setAsIdentityConversion();
    User.After.setFromType(ThisType->castAs<PointerType>()->getPointeeType());
    User.After.setAllToTypes(ToType);
    return Result;
  }

  case OR_No_Viable_Function:
    return OR_No_Viable_Function;
  case OR_Ambiguous:
    return OR_Ambiguous;
  }

  llvm_unreachable("Invalid OverloadResult!");
}

/// Determines whether there is a user-defined conversion sequence
/// (C++ [over.ics.user]) that converts expression From to the type
/// ToType. If such a conversion exists, User will contain the
/// user-defined conversion sequence that performs such a conversion
/// and this routine will return true. Otherwise, this routine returns
/// false and User is unspecified.
///
/// \param AllowExplicit  true if the conversion should consider C++0x
/// "explicit" conversion functions as well as non-explicit conversion
/// functions (C++0x [class.conv.fct]p2).
///
/// \param AllowObjCConversionOnExplicit true if the conversion should
/// allow an extra Objective-C pointer conversion on uses of explicit
/// constructors. Requires \c AllowExplicit to also be set.
static OverloadingResult
IsUserDefinedConversion(Sema &S, Expr *From, QualType ToType,
                        UserDefinedConversionSequence &User,
                        OverloadCandidateSet &CandidateSet,
                        AllowedExplicit AllowExplicit,
                        bool AllowObjCConversionOnExplicit) {
  assert(AllowExplicit != AllowedExplicit::None ||
         !AllowObjCConversionOnExplicit);
  CandidateSet.clear(OverloadCandidateSet::CSK_InitByUserDefinedConversion);

  // Whether we will only visit constructors.
  bool ConstructorsOnly = false;

  // If the type we are conversion to is a class type, enumerate its
  // constructors.
  if (const RecordType *ToRecordType = ToType->getAs<RecordType>()) {
    // C++ [over.match.ctor]p1:
    //   When objects of class type are direct-initialized (8.5), or
    //   copy-initialized from an expression of the same or a
    //   derived class type (8.5), overload resolution selects the
    //   constructor. [...] For copy-initialization, the candidate
    //   functions are all the converting constructors (12.3.1) of
    //   that class. The argument list is the expression-list within
    //   the parentheses of the initializer.
    if (S.Context.hasSameUnqualifiedType(ToType, From->getType()) ||
        (From->getType()->getAs<RecordType>() &&
         S.IsDerivedFrom(From->getBeginLoc(), From->getType(), ToType)))
      ConstructorsOnly = true;

    if (!S.isCompleteType(From->getExprLoc(), ToType)) {
      // We're not going to find any constructors.
    } else if (CXXRecordDecl *ToRecordDecl
                 = dyn_cast<CXXRecordDecl>(ToRecordType->getDecl())) {

      Expr **Args = &From;
      unsigned NumArgs = 1;
      bool ListInitializing = false;
      if (InitListExpr *InitList = dyn_cast<InitListExpr>(From)) {
        // But first, see if there is an init-list-constructor that will work.
        OverloadingResult Result = IsInitializerListConstructorConversion(
            S, From, ToType, ToRecordDecl, User, CandidateSet,
            AllowExplicit == AllowedExplicit::All);
        if (Result != OR_No_Viable_Function)
          return Result;
        // Never mind.
        CandidateSet.clear(
            OverloadCandidateSet::CSK_InitByUserDefinedConversion);

        // If we're list-initializing, we pass the individual elements as
        // arguments, not the entire list.
        Args = InitList->getInits();
        NumArgs = InitList->getNumInits();
        ListInitializing = true;
      }

      for (auto *D : S.LookupConstructors(ToRecordDecl)) {
        auto Info = getConstructorInfo(D);
        if (!Info)
          continue;

        bool Usable = !Info.Constructor->isInvalidDecl();
        if (!ListInitializing)
          Usable = Usable && Info.Constructor->isConvertingConstructor(
                                 /*AllowExplicit*/ true);
        if (Usable) {
          bool SuppressUserConversions = !ConstructorsOnly;
          // C++20 [over.best.ics.general]/4.5:
          //   if the target is the first parameter of a constructor [of class
          //   X] and the constructor [...] is a candidate by [...] the second
          //   phase of [over.match.list] when the initializer list has exactly
          //   one element that is itself an initializer list, [...] and the
          //   conversion is to X or reference to cv X, user-defined conversion
          //   sequences are not cnosidered.
          if (SuppressUserConversions && ListInitializing) {
            SuppressUserConversions =
                NumArgs == 1 && isa<InitListExpr>(Args[0]) &&
                isFirstArgumentCompatibleWithType(S.Context, Info.Constructor,
                                                  ToType);
          }
          if (Info.ConstructorTmpl)
            S.AddTemplateOverloadCandidate(
                Info.ConstructorTmpl, Info.FoundDecl,
                /*ExplicitArgs*/ nullptr, llvm::ArrayRef(Args, NumArgs),
                CandidateSet, SuppressUserConversions,
                /*PartialOverloading*/ false,
                AllowExplicit == AllowedExplicit::All);
          else
            // Allow one user-defined conversion when user specifies a
            // From->ToType conversion via an static cast (c-style, etc).
            S.AddOverloadCandidate(Info.Constructor, Info.FoundDecl,
                                   llvm::ArrayRef(Args, NumArgs), CandidateSet,
                                   SuppressUserConversions,
                                   /*PartialOverloading*/ false,
                                   AllowExplicit == AllowedExplicit::All);
        }
      }
    }
  }

  // Enumerate conversion functions, if we're allowed to.
  if (ConstructorsOnly || isa<InitListExpr>(From)) {
  } else if (!S.isCompleteType(From->getBeginLoc(), From->getType())) {
    // No conversion functions from incomplete types.
  } else if (const RecordType *FromRecordType =
                 From->getType()->getAs<RecordType>()) {
    if (CXXRecordDecl *FromRecordDecl
         = dyn_cast<CXXRecordDecl>(FromRecordType->getDecl())) {
      // Add all of the conversion functions as candidates.
      const auto &Conversions = FromRecordDecl->getVisibleConversionFunctions();
      for (auto I = Conversions.begin(), E = Conversions.end(); I != E; ++I) {
        DeclAccessPair FoundDecl = I.getPair();
        NamedDecl *D = FoundDecl.getDecl();
        CXXRecordDecl *ActingContext = cast<CXXRecordDecl>(D->getDeclContext());
        if (isa<UsingShadowDecl>(D))
          D = cast<UsingShadowDecl>(D)->getTargetDecl();

        CXXConversionDecl *Conv;
        FunctionTemplateDecl *ConvTemplate;
        if ((ConvTemplate = dyn_cast<FunctionTemplateDecl>(D)))
          Conv = cast<CXXConversionDecl>(ConvTemplate->getTemplatedDecl());
        else
          Conv = cast<CXXConversionDecl>(D);

        if (ConvTemplate)
          S.AddTemplateConversionCandidate(
              ConvTemplate, FoundDecl, ActingContext, From, ToType,
              CandidateSet, AllowObjCConversionOnExplicit,
              AllowExplicit != AllowedExplicit::None);
        else
          S.AddConversionCandidate(Conv, FoundDecl, ActingContext, From, ToType,
                                   CandidateSet, AllowObjCConversionOnExplicit,
                                   AllowExplicit != AllowedExplicit::None);
      }
    }
  }

  bool HadMultipleCandidates = (CandidateSet.size() > 1);

  OverloadCandidateSet::iterator Best;
  switch (auto Result =
              CandidateSet.BestViableFunction(S, From->getBeginLoc(), Best)) {
  case OR_Success:
  case OR_Deleted:
    // Record the standard conversion we used and the conversion function.
    if (CXXConstructorDecl *Constructor
          = dyn_cast<CXXConstructorDecl>(Best->Function)) {
      // C++ [over.ics.user]p1:
      //   If the user-defined conversion is specified by a
      //   constructor (12.3.1), the initial standard conversion
      //   sequence converts the source type to the type required by
      //   the argument of the constructor.
      //
      QualType ThisType = Constructor->getThisType();
      if (isa<InitListExpr>(From)) {
        // Initializer lists don't have conversions as such.
        User.Before.setAsIdentityConversion();
      } else {
        if (Best->Conversions[0].isEllipsis())
          User.EllipsisConversion = true;
        else {
          User.Before = Best->Conversions[0].Standard;
          User.EllipsisConversion = false;
        }
      }
      User.HadMultipleCandidates = HadMultipleCandidates;
      User.ConversionFunction = Constructor;
      User.FoundConversionFunction = Best->FoundDecl;
      User.After.setAsIdentityConversion();
      User.After.setFromType(ThisType->castAs<PointerType>()->getPointeeType());
      User.After.setAllToTypes(ToType);
      return Result;
    }
    if (CXXConversionDecl *Conversion
                 = dyn_cast<CXXConversionDecl>(Best->Function)) {
      // C++ [over.ics.user]p1:
      //
      //   [...] If the user-defined conversion is specified by a
      //   conversion function (12.3.2), the initial standard
      //   conversion sequence converts the source type to the
      //   implicit object parameter of the conversion function.
      User.Before = Best->Conversions[0].Standard;
      User.HadMultipleCandidates = HadMultipleCandidates;
      User.ConversionFunction = Conversion;
      User.FoundConversionFunction = Best->FoundDecl;
      User.EllipsisConversion = false;

      // C++ [over.ics.user]p2:
      //   The second standard conversion sequence converts the
      //   result of the user-defined conversion to the target type
      //   for the sequence. Since an implicit conversion sequence
      //   is an initialization, the special rules for
      //   initialization by user-defined conversion apply when
      //   selecting the best user-defined conversion for a
      //   user-defined conversion sequence (see 13.3.3 and
      //   13.3.3.1).
      User.After = Best->FinalConversion;
      return Result;
    }
    llvm_unreachable("Not a constructor or conversion function?");

  case OR_No_Viable_Function:
    return OR_No_Viable_Function;

  case OR_Ambiguous:
    return OR_Ambiguous;
  }

  llvm_unreachable("Invalid OverloadResult!");
}

bool
Sema::DiagnoseMultipleUserDefinedConversion(Expr *From, QualType ToType) {
  ImplicitConversionSequence ICS;
  OverloadCandidateSet CandidateSet(From->getExprLoc(),
                                    OverloadCandidateSet::CSK_Normal);
  OverloadingResult OvResult =
    IsUserDefinedConversion(*this, From, ToType, ICS.UserDefined,
                            CandidateSet, AllowedExplicit::None, false);

  if (!(OvResult == OR_Ambiguous ||
        (OvResult == OR_No_Viable_Function && !CandidateSet.empty())))
    return false;

  auto Cands = CandidateSet.CompleteCandidates(
      *this,
      OvResult == OR_Ambiguous ? OCD_AmbiguousCandidates : OCD_AllCandidates,
      From);
  if (OvResult == OR_Ambiguous)
    Diag(From->getBeginLoc(), diag::err_typecheck_ambiguous_condition)
        << From->getType() << ToType << From->getSourceRange();
  else { // OR_No_Viable_Function && !CandidateSet.empty()
    if (!RequireCompleteType(From->getBeginLoc(), ToType,
                             diag::err_typecheck_nonviable_condition_incomplete,
                             From->getType(), From->getSourceRange()))
      Diag(From->getBeginLoc(), diag::err_typecheck_nonviable_condition)
          << false << From->getType() << From->getSourceRange() << ToType;
  }

  CandidateSet.NoteCandidates(
                              *this, From, Cands);
  return true;
}

// Helper for compareConversionFunctions that gets the FunctionType that the
// conversion-operator return  value 'points' to, or nullptr.
static const FunctionType *
getConversionOpReturnTyAsFunction(CXXConversionDecl *Conv) {
  const FunctionType *ConvFuncTy = Conv->getType()->castAs<FunctionType>();
  const PointerType *RetPtrTy =
      ConvFuncTy->getReturnType()->getAs<PointerType>();

  if (!RetPtrTy)
    return nullptr;

  return RetPtrTy->getPointeeType()->getAs<FunctionType>();
}

/// Compare the user-defined conversion functions or constructors
/// of two user-defined conversion sequences to determine whether any ordering
/// is possible.
static ImplicitConversionSequence::CompareKind
compareConversionFunctions(Sema &S, FunctionDecl *Function1,
                           FunctionDecl *Function2) {
  CXXConversionDecl *Conv1 = dyn_cast_or_null<CXXConversionDecl>(Function1);
  CXXConversionDecl *Conv2 = dyn_cast_or_null<CXXConversionDecl>(Function2);
  if (!Conv1 || !Conv2)
    return ImplicitConversionSequence::Indistinguishable;

  if (!Conv1->getParent()->isLambda() || !Conv2->getParent()->isLambda())
    return ImplicitConversionSequence::Indistinguishable;

  // Objective-C++:
  //   If both conversion functions are implicitly-declared conversions from
  //   a lambda closure type to a function pointer and a block pointer,
  //   respectively, always prefer the conversion to a function pointer,
  //   because the function pointer is more lightweight and is more likely
  //   to keep code working.
  if (S.getLangOpts().ObjC && S.getLangOpts().CPlusPlus11) {
    bool Block1 = Conv1->getConversionType()->isBlockPointerType();
    bool Block2 = Conv2->getConversionType()->isBlockPointerType();
    if (Block1 != Block2)
      return Block1 ? ImplicitConversionSequence::Worse
                    : ImplicitConversionSequence::Better;
  }

  // In order to support multiple calling conventions for the lambda conversion
  // operator (such as when the free and member function calling convention is
  // different), prefer the 'free' mechanism, followed by the calling-convention
  // of operator(). The latter is in place to support the MSVC-like solution of
  // defining ALL of the possible conversions in regards to calling-convention.
  const FunctionType *Conv1FuncRet = getConversionOpReturnTyAsFunction(Conv1);
  const FunctionType *Conv2FuncRet = getConversionOpReturnTyAsFunction(Conv2);

  if (Conv1FuncRet && Conv2FuncRet &&
      Conv1FuncRet->getCallConv() != Conv2FuncRet->getCallConv()) {
    CallingConv Conv1CC = Conv1FuncRet->getCallConv();
    CallingConv Conv2CC = Conv2FuncRet->getCallConv();

    CXXMethodDecl *CallOp = Conv2->getParent()->getLambdaCallOperator();
    const auto *CallOpProto = CallOp->getType()->castAs<FunctionProtoType>();

    CallingConv CallOpCC =
        CallOp->getType()->castAs<FunctionType>()->getCallConv();
    CallingConv DefaultFree = S.Context.getDefaultCallingConvention(
        CallOpProto->isVariadic(), /*IsCXXMethod=*/false);
    CallingConv DefaultMember = S.Context.getDefaultCallingConvention(
        CallOpProto->isVariadic(), /*IsCXXMethod=*/true);

    CallingConv PrefOrder[] = {DefaultFree, DefaultMember, CallOpCC};
    for (CallingConv CC : PrefOrder) {
      if (Conv1CC == CC)
        return ImplicitConversionSequence::Better;
      if (Conv2CC == CC)
        return ImplicitConversionSequence::Worse;
    }
  }

  return ImplicitConversionSequence::Indistinguishable;
}

static bool hasDeprecatedStringLiteralToCharPtrConversion(
    const ImplicitConversionSequence &ICS) {
  return (ICS.isStandard() && ICS.Standard.DeprecatedStringLiteralToCharPtr) ||
         (ICS.isUserDefined() &&
          ICS.UserDefined.Before.DeprecatedStringLiteralToCharPtr);
}

/// CompareImplicitConversionSequences - Compare two implicit
/// conversion sequences to determine whether one is better than the
/// other or if they are indistinguishable (C++ 13.3.3.2).
static ImplicitConversionSequence::CompareKind
CompareImplicitConversionSequences(Sema &S, SourceLocation Loc,
                                   const ImplicitConversionSequence& ICS1,
                                   const ImplicitConversionSequence& ICS2)
{
  // (C++ 13.3.3.2p2): When comparing the basic forms of implicit
  // conversion sequences (as defined in 13.3.3.1)
  //   -- a standard conversion sequence (13.3.3.1.1) is a better
  //      conversion sequence than a user-defined conversion sequence or
  //      an ellipsis conversion sequence, and
  //   -- a user-defined conversion sequence (13.3.3.1.2) is a better
  //      conversion sequence than an ellipsis conversion sequence
  //      (13.3.3.1.3).
  //
  // C++0x [over.best.ics]p10:
  //   For the purpose of ranking implicit conversion sequences as
  //   described in 13.3.3.2, the ambiguous conversion sequence is
  //   treated as a user-defined sequence that is indistinguishable
  //   from any other user-defined conversion sequence.

  // String literal to 'char *' conversion has been deprecated in C++03. It has
  // been removed from C++11. We still accept this conversion, if it happens at
  // the best viable function. Otherwise, this conversion is considered worse
  // than ellipsis conversion. Consider this as an extension; this is not in the
  // standard. For example:
  //
  // int &f(...);    // #1
  // void f(char*);  // #2
  // void g() { int &r = f("foo"); }
  //
  // In C++03, we pick #2 as the best viable function.
  // In C++11, we pick #1 as the best viable function, because ellipsis
  // conversion is better than string-literal to char* conversion (since there
  // is no such conversion in C++11). If there was no #1 at all or #1 couldn't
  // convert arguments, #2 would be the best viable function in C++11.
  // If the best viable function has this conversion, a warning will be issued
  // in C++03, or an ExtWarn (+SFINAE failure) will be issued in C++11.

  if (S.getLangOpts().CPlusPlus11 && !S.getLangOpts().WritableStrings &&
      hasDeprecatedStringLiteralToCharPtrConversion(ICS1) !=
          hasDeprecatedStringLiteralToCharPtrConversion(ICS2) &&
      // Ill-formedness must not differ
      ICS1.isBad() == ICS2.isBad())
    return hasDeprecatedStringLiteralToCharPtrConversion(ICS1)
               ? ImplicitConversionSequence::Worse
               : ImplicitConversionSequence::Better;

  if (ICS1.getKindRank() < ICS2.getKindRank())
    return ImplicitConversionSequence::Better;
  if (ICS2.getKindRank() < ICS1.getKindRank())
    return ImplicitConversionSequence::Worse;

  // The following checks require both conversion sequences to be of
  // the same kind.
  if (ICS1.getKind() != ICS2.getKind())
    return ImplicitConversionSequence::Indistinguishable;

  ImplicitConversionSequence::CompareKind Result =
      ImplicitConversionSequence::Indistinguishable;

  // Two implicit conversion sequences of the same form are
  // indistinguishable conversion sequences unless one of the
  // following rules apply: (C++ 13.3.3.2p3):

  // List-initialization sequence L1 is a better conversion sequence than
  // list-initialization sequence L2 if:
  // - L1 converts to std::initializer_list<X> for some X and L2 does not, or,
  //   if not that,
  // — L1 and L2 convert to arrays of the same element type, and either the
  //   number of elements n_1 initialized by L1 is less than the number of
  //   elements n_2 initialized by L2, or (C++20) n_1 = n_2 and L2 converts to
  //   an array of unknown bound and L1 does not,
  // even if one of the other rules in this paragraph would otherwise apply.
  if (!ICS1.isBad()) {
    bool StdInit1 = false, StdInit2 = false;
    if (ICS1.hasInitializerListContainerType())
      StdInit1 = S.isStdInitializerList(ICS1.getInitializerListContainerType(),
                                        nullptr);
    if (ICS2.hasInitializerListContainerType())
      StdInit2 = S.isStdInitializerList(ICS2.getInitializerListContainerType(),
                                        nullptr);
    if (StdInit1 != StdInit2)
      return StdInit1 ? ImplicitConversionSequence::Better
                      : ImplicitConversionSequence::Worse;

    if (ICS1.hasInitializerListContainerType() &&
        ICS2.hasInitializerListContainerType())
      if (auto *CAT1 = S.Context.getAsConstantArrayType(
              ICS1.getInitializerListContainerType()))
        if (auto *CAT2 = S.Context.getAsConstantArrayType(
                ICS2.getInitializerListContainerType())) {
          if (S.Context.hasSameUnqualifiedType(CAT1->getElementType(),
                                               CAT2->getElementType())) {
            // Both to arrays of the same element type
            if (CAT1->getSize() != CAT2->getSize())
              // Different sized, the smaller wins
              return CAT1->getSize().ult(CAT2->getSize())
                         ? ImplicitConversionSequence::Better
                         : ImplicitConversionSequence::Worse;
            if (ICS1.isInitializerListOfIncompleteArray() !=
                ICS2.isInitializerListOfIncompleteArray())
              // One is incomplete, it loses
              return ICS2.isInitializerListOfIncompleteArray()
                         ? ImplicitConversionSequence::Better
                         : ImplicitConversionSequence::Worse;
          }
        }
  }

  if (ICS1.isStandard())
    // Standard conversion sequence S1 is a better conversion sequence than
    // standard conversion sequence S2 if [...]
    Result = CompareStandardConversionSequences(S, Loc,
                                                ICS1.Standard, ICS2.Standard);
  else if (ICS1.isUserDefined()) {
    // User-defined conversion sequence U1 is a better conversion
    // sequence than another user-defined conversion sequence U2 if
    // they contain the same user-defined conversion function or
    // constructor and if the second standard conversion sequence of
    // U1 is better than the second standard conversion sequence of
    // U2 (C++ 13.3.3.2p3).
    if (ICS1.UserDefined.ConversionFunction ==
          ICS2.UserDefined.ConversionFunction)
      Result = CompareStandardConversionSequences(S, Loc,
                                                  ICS1.UserDefined.After,
                                                  ICS2.UserDefined.After);
    else
      Result = compareConversionFunctions(S,
                                          ICS1.UserDefined.ConversionFunction,
                                          ICS2.UserDefined.ConversionFunction);
  }

  return Result;
}

// Per 13.3.3.2p3, compare the given standard conversion sequences to
// determine if one is a proper subset of the other.
static ImplicitConversionSequence::CompareKind
compareStandardConversionSubsets(ASTContext &Context,
                                 const StandardConversionSequence& SCS1,
                                 const StandardConversionSequence& SCS2) {
  ImplicitConversionSequence::CompareKind Result
    = ImplicitConversionSequence::Indistinguishable;

  // the identity conversion sequence is considered to be a subsequence of
  // any non-identity conversion sequence
  if (SCS1.isIdentityConversion() && !SCS2.isIdentityConversion())
    return ImplicitConversionSequence::Better;
  else if (!SCS1.isIdentityConversion() && SCS2.isIdentityConversion())
    return ImplicitConversionSequence::Worse;

  if (SCS1.Second != SCS2.Second) {
    if (SCS1.Second == ICK_Identity)
      Result = ImplicitConversionSequence::Better;
    else if (SCS2.Second == ICK_Identity)
      Result = ImplicitConversionSequence::Worse;
    else
      return ImplicitConversionSequence::Indistinguishable;
  } else if (!Context.hasSimilarType(SCS1.getToType(1), SCS2.getToType(1)))
    return ImplicitConversionSequence::Indistinguishable;

  if (SCS1.Third == SCS2.Third) {
    return Context.hasSameType(SCS1.getToType(2), SCS2.getToType(2))? Result
                             : ImplicitConversionSequence::Indistinguishable;
  }

  if (SCS1.Third == ICK_Identity)
    return Result == ImplicitConversionSequence::Worse
             ? ImplicitConversionSequence::Indistinguishable
             : ImplicitConversionSequence::Better;

  if (SCS2.Third == ICK_Identity)
    return Result == ImplicitConversionSequence::Better
             ? ImplicitConversionSequence::Indistinguishable
             : ImplicitConversionSequence::Worse;

  return ImplicitConversionSequence::Indistinguishable;
}

/// Determine whether one of the given reference bindings is better
/// than the other based on what kind of bindings they are.
static bool
isBetterReferenceBindingKind(const StandardConversionSequence &SCS1,
                             const StandardConversionSequence &SCS2) {
  // C++0x [over.ics.rank]p3b4:
  //   -- S1 and S2 are reference bindings (8.5.3) and neither refers to an
  //      implicit object parameter of a non-static member function declared
  //      without a ref-qualifier, and *either* S1 binds an rvalue reference
  //      to an rvalue and S2 binds an lvalue reference *or S1 binds an
  //      lvalue reference to a function lvalue and S2 binds an rvalue
  //      reference*.
  //
  // FIXME: Rvalue references. We're going rogue with the above edits,
  // because the semantics in the current C++0x working paper (N3225 at the
  // time of this writing) break the standard definition of std::forward
  // and std::reference_wrapper when dealing with references to functions.
  // Proposed wording changes submitted to CWG for consideration.
  if (SCS1.BindsImplicitObjectArgumentWithoutRefQualifier ||
      SCS2.BindsImplicitObjectArgumentWithoutRefQualifier)
    return false;

  return (!SCS1.IsLvalueReference && SCS1.BindsToRvalue &&
          SCS2.IsLvalueReference) ||
         (SCS1.IsLvalueReference && SCS1.BindsToFunctionLvalue &&
          !SCS2.IsLvalueReference && SCS2.BindsToFunctionLvalue);
}

enum class FixedEnumPromotion {
  None,
  ToUnderlyingType,
  ToPromotedUnderlyingType
};

/// Returns kind of fixed enum promotion the \a SCS uses.
static FixedEnumPromotion
getFixedEnumPromtion(Sema &S, const StandardConversionSequence &SCS) {

  if (SCS.Second != ICK_Integral_Promotion)
    return FixedEnumPromotion::None;

  QualType FromType = SCS.getFromType();
  if (!FromType->isEnumeralType())
    return FixedEnumPromotion::None;

  EnumDecl *Enum = FromType->castAs<EnumType>()->getDecl();
  if (!Enum->isFixed())
    return FixedEnumPromotion::None;

  QualType UnderlyingType = Enum->getIntegerType();
  if (S.Context.hasSameType(SCS.getToType(1), UnderlyingType))
    return FixedEnumPromotion::ToUnderlyingType;

  return FixedEnumPromotion::ToPromotedUnderlyingType;
}

/// CompareStandardConversionSequences - Compare two standard
/// conversion sequences to determine whether one is better than the
/// other or if they are indistinguishable (C++ 13.3.3.2p3).
static ImplicitConversionSequence::CompareKind
CompareStandardConversionSequences(Sema &S, SourceLocation Loc,
                                   const StandardConversionSequence& SCS1,
                                   const StandardConversionSequence& SCS2)
{
  // Standard conversion sequence S1 is a better conversion sequence
  // than standard conversion sequence S2 if (C++ 13.3.3.2p3):

  //  -- S1 is a proper subsequence of S2 (comparing the conversion
  //     sequences in the canonical form defined by 13.3.3.1.1,
  //     excluding any Lvalue Transformation; the identity conversion
  //     sequence is considered to be a subsequence of any
  //     non-identity conversion sequence) or, if not that,
  if (ImplicitConversionSequence::CompareKind CK
        = compareStandardConversionSubsets(S.Context, SCS1, SCS2))
    return CK;

  //  -- the rank of S1 is better than the rank of S2 (by the rules
  //     defined below), or, if not that,
  ImplicitConversionRank Rank1 = SCS1.getRank();
  ImplicitConversionRank Rank2 = SCS2.getRank();
  if (Rank1 < Rank2)
    return ImplicitConversionSequence::Better;
  else if (Rank2 < Rank1)
    return ImplicitConversionSequence::Worse;

  // (C++ 13.3.3.2p4): Two conversion sequences with the same rank
  // are indistinguishable unless one of the following rules
  // applies:

  //   A conversion that is not a conversion of a pointer, or
  //   pointer to member, to bool is better than another conversion
  //   that is such a conversion.
  if (SCS1.isPointerConversionToBool() != SCS2.isPointerConversionToBool())
    return SCS2.isPointerConversionToBool()
             ? ImplicitConversionSequence::Better
             : ImplicitConversionSequence::Worse;

  // C++14 [over.ics.rank]p4b2:
  // This is retroactively applied to C++11 by CWG 1601.
  //
  //   A conversion that promotes an enumeration whose underlying type is fixed
  //   to its underlying type is better than one that promotes to the promoted
  //   underlying type, if the two are different.
  FixedEnumPromotion FEP1 = getFixedEnumPromtion(S, SCS1);
  FixedEnumPromotion FEP2 = getFixedEnumPromtion(S, SCS2);
  if (FEP1 != FixedEnumPromotion::None && FEP2 != FixedEnumPromotion::None &&
      FEP1 != FEP2)
    return FEP1 == FixedEnumPromotion::ToUnderlyingType
               ? ImplicitConversionSequence::Better
               : ImplicitConversionSequence::Worse;

  // C++ [over.ics.rank]p4b2:
  //
  //   If class B is derived directly or indirectly from class A,
  //   conversion of B* to A* is better than conversion of B* to
  //   void*, and conversion of A* to void* is better than conversion
  //   of B* to void*.
  bool SCS1ConvertsToVoid
    = SCS1.isPointerConversionToVoidPointer(S.Context);
  bool SCS2ConvertsToVoid
    = SCS2.isPointerConversionToVoidPointer(S.Context);
  if (SCS1ConvertsToVoid != SCS2ConvertsToVoid) {
    // Exactly one of the conversion sequences is a conversion to
    // a void pointer; it's the worse conversion.
    return SCS2ConvertsToVoid ? ImplicitConversionSequence::Better
                              : ImplicitConversionSequence::Worse;
  } else if (!SCS1ConvertsToVoid && !SCS2ConvertsToVoid) {
    // Neither conversion sequence converts to a void pointer; compare
    // their derived-to-base conversions.
    if (ImplicitConversionSequence::CompareKind DerivedCK
          = CompareDerivedToBaseConversions(S, Loc, SCS1, SCS2))
      return DerivedCK;
  } else if (SCS1ConvertsToVoid && SCS2ConvertsToVoid &&
             !S.Context.hasSameType(SCS1.getFromType(), SCS2.getFromType())) {
    // Both conversion sequences are conversions to void
    // pointers. Compare the source types to determine if there's an
    // inheritance relationship in their sources.
    QualType FromType1 = SCS1.getFromType();
    QualType FromType2 = SCS2.getFromType();

    // Adjust the types we're converting from via the array-to-pointer
    // conversion, if we need to.
    if (SCS1.First == ICK_Array_To_Pointer)
      FromType1 = S.Context.getArrayDecayedType(FromType1);
    if (SCS2.First == ICK_Array_To_Pointer)
      FromType2 = S.Context.getArrayDecayedType(FromType2);

    QualType FromPointee1 = FromType1->getPointeeType().getUnqualifiedType();
    QualType FromPointee2 = FromType2->getPointeeType().getUnqualifiedType();

    if (S.IsDerivedFrom(Loc, FromPointee2, FromPointee1))
      return ImplicitConversionSequence::Better;
    else if (S.IsDerivedFrom(Loc, FromPointee1, FromPointee2))
      return ImplicitConversionSequence::Worse;

    // Objective-C++: If one interface is more specific than the
    // other, it is the better one.
    const ObjCObjectPointerType* FromObjCPtr1
      = FromType1->getAs<ObjCObjectPointerType>();
    const ObjCObjectPointerType* FromObjCPtr2
      = FromType2->getAs<ObjCObjectPointerType>();
    if (FromObjCPtr1 && FromObjCPtr2) {
      bool AssignLeft = S.Context.canAssignObjCInterfaces(FromObjCPtr1,
                                                          FromObjCPtr2);
      bool AssignRight = S.Context.canAssignObjCInterfaces(FromObjCPtr2,
                                                           FromObjCPtr1);
      if (AssignLeft != AssignRight) {
        return AssignLeft? ImplicitConversionSequence::Better
                         : ImplicitConversionSequence::Worse;
      }
    }
  }

  if (SCS1.ReferenceBinding && SCS2.ReferenceBinding) {
    // Check for a better reference binding based on the kind of bindings.
    if (isBetterReferenceBindingKind(SCS1, SCS2))
      return ImplicitConversionSequence::Better;
    else if (isBetterReferenceBindingKind(SCS2, SCS1))
      return ImplicitConversionSequence::Worse;
  }

  // Compare based on qualification conversions (C++ 13.3.3.2p3,
  // bullet 3).
  if (ImplicitConversionSequence::CompareKind QualCK
        = CompareQualificationConversions(S, SCS1, SCS2))
    return QualCK;

  if (SCS1.ReferenceBinding && SCS2.ReferenceBinding) {
    // C++ [over.ics.rank]p3b4:
    //   -- S1 and S2 are reference bindings (8.5.3), and the types to
    //      which the references refer are the same type except for
    //      top-level cv-qualifiers, and the type to which the reference
    //      initialized by S2 refers is more cv-qualified than the type
    //      to which the reference initialized by S1 refers.
    QualType T1 = SCS1.getToType(2);
    QualType T2 = SCS2.getToType(2);
    T1 = S.Context.getCanonicalType(T1);
    T2 = S.Context.getCanonicalType(T2);
    Qualifiers T1Quals, T2Quals;
    QualType UnqualT1 = S.Context.getUnqualifiedArrayType(T1, T1Quals);
    QualType UnqualT2 = S.Context.getUnqualifiedArrayType(T2, T2Quals);
    if (UnqualT1 == UnqualT2) {
      // Objective-C++ ARC: If the references refer to objects with different
      // lifetimes, prefer bindings that don't change lifetime.
      if (SCS1.ObjCLifetimeConversionBinding !=
                                          SCS2.ObjCLifetimeConversionBinding) {
        return SCS1.ObjCLifetimeConversionBinding
                                           ? ImplicitConversionSequence::Worse
                                           : ImplicitConversionSequence::Better;
      }

      // If the type is an array type, promote the element qualifiers to the
      // type for comparison.
      if (isa<ArrayType>(T1) && T1Quals)
        T1 = S.Context.getQualifiedType(UnqualT1, T1Quals);
      if (isa<ArrayType>(T2) && T2Quals)
        T2 = S.Context.getQualifiedType(UnqualT2, T2Quals);
      if (T2.isMoreQualifiedThan(T1))
        return ImplicitConversionSequence::Better;
      if (T1.isMoreQualifiedThan(T2))
        return ImplicitConversionSequence::Worse;
    }
  }

  // In Microsoft mode (below 19.28), prefer an integral conversion to a
  // floating-to-integral conversion if the integral conversion
  // is between types of the same size.
  // For example:
  // void f(float);
  // void f(int);
  // int main {
  //    long a;
  //    f(a);
  // }
  // Here, MSVC will call f(int) instead of generating a compile error
  // as clang will do in standard mode.
  if (S.getLangOpts().MSVCCompat &&
      !S.getLangOpts().isCompatibleWithMSVC(LangOptions::MSVC2019_8) &&
      SCS1.Second == ICK_Integral_Conversion &&
      SCS2.Second == ICK_Floating_Integral &&
      S.Context.getTypeSize(SCS1.getFromType()) ==
          S.Context.getTypeSize(SCS1.getToType(2)))
    return ImplicitConversionSequence::Better;

  // Prefer a compatible vector conversion over a lax vector conversion
  // For example:
  //
  // typedef float __v4sf __attribute__((__vector_size__(16)));
  // void f(vector float);
  // void f(vector signed int);
  // int main() {
  //   __v4sf a;
  //   f(a);
  // }
  // Here, we'd like to choose f(vector float) and not
  // report an ambiguous call error
  if (SCS1.Second == ICK_Vector_Conversion &&
      SCS2.Second == ICK_Vector_Conversion) {
    bool SCS1IsCompatibleVectorConversion = S.Context.areCompatibleVectorTypes(
        SCS1.getFromType(), SCS1.getToType(2));
    bool SCS2IsCompatibleVectorConversion = S.Context.areCompatibleVectorTypes(
        SCS2.getFromType(), SCS2.getToType(2));

    if (SCS1IsCompatibleVectorConversion != SCS2IsCompatibleVectorConversion)
      return SCS1IsCompatibleVectorConversion
                 ? ImplicitConversionSequence::Better
                 : ImplicitConversionSequence::Worse;
  }

  if (SCS1.Second == ICK_SVE_Vector_Conversion &&
      SCS2.Second == ICK_SVE_Vector_Conversion) {
    bool SCS1IsCompatibleSVEVectorConversion =
        S.Context.areCompatibleSveTypes(SCS1.getFromType(), SCS1.getToType(2));
    bool SCS2IsCompatibleSVEVectorConversion =
        S.Context.areCompatibleSveTypes(SCS2.getFromType(), SCS2.getToType(2));

    if (SCS1IsCompatibleSVEVectorConversion !=
        SCS2IsCompatibleSVEVectorConversion)
      return SCS1IsCompatibleSVEVectorConversion
                 ? ImplicitConversionSequence::Better
                 : ImplicitConversionSequence::Worse;
  }

  if (SCS1.Second == ICK_RVV_Vector_Conversion &&
      SCS2.Second == ICK_RVV_Vector_Conversion) {
    bool SCS1IsCompatibleRVVVectorConversion =
        S.Context.areCompatibleRVVTypes(SCS1.getFromType(), SCS1.getToType(2));
    bool SCS2IsCompatibleRVVVectorConversion =
        S.Context.areCompatibleRVVTypes(SCS2.getFromType(), SCS2.getToType(2));

    if (SCS1IsCompatibleRVVVectorConversion !=
        SCS2IsCompatibleRVVVectorConversion)
      return SCS1IsCompatibleRVVVectorConversion
                 ? ImplicitConversionSequence::Better
                 : ImplicitConversionSequence::Worse;
  }

  return ImplicitConversionSequence::Indistinguishable;
}

/// CompareQualificationConversions - Compares two standard conversion
/// sequences to determine whether they can be ranked based on their
/// qualification conversions (C++ 13.3.3.2p3 bullet 3).
static ImplicitConversionSequence::CompareKind
CompareQualificationConversions(Sema &S,
                                const StandardConversionSequence& SCS1,
                                const StandardConversionSequence& SCS2) {
  // C++ [over.ics.rank]p3:
  //  -- S1 and S2 differ only in their qualification conversion and
  //     yield similar types T1 and T2 (C++ 4.4), respectively, [...]
  // [C++98]
  //     [...] and the cv-qualification signature of type T1 is a proper subset
  //     of the cv-qualification signature of type T2, and S1 is not the
  //     deprecated string literal array-to-pointer conversion (4.2).
  // [C++2a]
  //     [...] where T1 can be converted to T2 by a qualification conversion.
  if (SCS1.First != SCS2.First || SCS1.Second != SCS2.Second ||
      SCS1.Third != SCS2.Third || SCS1.Third != ICK_Qualification)
    return ImplicitConversionSequence::Indistinguishable;

  // FIXME: the example in the standard doesn't use a qualification
  // conversion (!)
  QualType T1 = SCS1.getToType(2);
  QualType T2 = SCS2.getToType(2);
  T1 = S.Context.getCanonicalType(T1);
  T2 = S.Context.getCanonicalType(T2);
  assert(!T1->isReferenceType() && !T2->isReferenceType());
  Qualifiers T1Quals, T2Quals;
  QualType UnqualT1 = S.Context.getUnqualifiedArrayType(T1, T1Quals);
  QualType UnqualT2 = S.Context.getUnqualifiedArrayType(T2, T2Quals);

  // If the types are the same, we won't learn anything by unwrapping
  // them.
  if (UnqualT1 == UnqualT2)
    return ImplicitConversionSequence::Indistinguishable;

  // Don't ever prefer a standard conversion sequence that uses the deprecated
  // string literal array to pointer conversion.
  bool CanPick1 = !SCS1.DeprecatedStringLiteralToCharPtr;
  bool CanPick2 = !SCS2.DeprecatedStringLiteralToCharPtr;

  // Objective-C++ ARC:
  //   Prefer qualification conversions not involving a change in lifetime
  //   to qualification conversions that do change lifetime.
  if (SCS1.QualificationIncludesObjCLifetime &&
      !SCS2.QualificationIncludesObjCLifetime)
    CanPick1 = false;
  if (SCS2.QualificationIncludesObjCLifetime &&
      !SCS1.QualificationIncludesObjCLifetime)
    CanPick2 = false;

  bool ObjCLifetimeConversion;
  if (CanPick1 &&
      !S.IsQualificationConversion(T1, T2, false, ObjCLifetimeConversion))
    CanPick1 = false;
  // FIXME: In Objective-C ARC, we can have qualification conversions in both
  // directions, so we can't short-cut this second check in general.
  if (CanPick2 &&
      !S.IsQualificationConversion(T2, T1, false, ObjCLifetimeConversion))
    CanPick2 = false;

  if (CanPick1 != CanPick2)
    return CanPick1 ? ImplicitConversionSequence::Better
                    : ImplicitConversionSequence::Worse;
  return ImplicitConversionSequence::Indistinguishable;
}

/// CompareDerivedToBaseConversions - Compares two standard conversion
/// sequences to determine whether they can be ranked based on their
/// various kinds of derived-to-base conversions (C++
/// [over.ics.rank]p4b3).  As part of these checks, we also look at
/// conversions between Objective-C interface types.
static ImplicitConversionSequence::CompareKind
CompareDerivedToBaseConversions(Sema &S, SourceLocation Loc,
                                const StandardConversionSequence& SCS1,
                                const StandardConversionSequence& SCS2) {
  QualType FromType1 = SCS1.getFromType();
  QualType ToType1 = SCS1.getToType(1);
  QualType FromType2 = SCS2.getFromType();
  QualType ToType2 = SCS2.getToType(1);

  // Adjust the types we're converting from via the array-to-pointer
  // conversion, if we need to.
  if (SCS1.First == ICK_Array_To_Pointer)
    FromType1 = S.Context.getArrayDecayedType(FromType1);
  if (SCS2.First == ICK_Array_To_Pointer)
    FromType2 = S.Context.getArrayDecayedType(FromType2);

  // Canonicalize all of the types.
  FromType1 = S.Context.getCanonicalType(FromType1);
  ToType1 = S.Context.getCanonicalType(ToType1);
  FromType2 = S.Context.getCanonicalType(FromType2);
  ToType2 = S.Context.getCanonicalType(ToType2);

  // C++ [over.ics.rank]p4b3:
  //
  //   If class B is derived directly or indirectly from class A and
  //   class C is derived directly or indirectly from B,
  //
  // Compare based on pointer conversions.
  if (SCS1.Second == ICK_Pointer_Conversion &&
      SCS2.Second == ICK_Pointer_Conversion &&
      /*FIXME: Remove if Objective-C id conversions get their own rank*/
      FromType1->isPointerType() && FromType2->isPointerType() &&
      ToType1->isPointerType() && ToType2->isPointerType()) {
    QualType FromPointee1 =
        FromType1->castAs<PointerType>()->getPointeeType().getUnqualifiedType();
    QualType ToPointee1 =
        ToType1->castAs<PointerType>()->getPointeeType().getUnqualifiedType();
    QualType FromPointee2 =
        FromType2->castAs<PointerType>()->getPointeeType().getUnqualifiedType();
    QualType ToPointee2 =
        ToType2->castAs<PointerType>()->getPointeeType().getUnqualifiedType();

    //   -- conversion of C* to B* is better than conversion of C* to A*,
    if (FromPointee1 == FromPointee2 && ToPointee1 != ToPointee2) {
      if (S.IsDerivedFrom(Loc, ToPointee1, ToPointee2))
        return ImplicitConversionSequence::Better;
      else if (S.IsDerivedFrom(Loc, ToPointee2, ToPointee1))
        return ImplicitConversionSequence::Worse;
    }

    //   -- conversion of B* to A* is better than conversion of C* to A*,
    if (FromPointee1 != FromPointee2 && ToPointee1 == ToPointee2) {
      if (S.IsDerivedFrom(Loc, FromPointee2, FromPointee1))
        return ImplicitConversionSequence::Better;
      else if (S.IsDerivedFrom(Loc, FromPointee1, FromPointee2))
        return ImplicitConversionSequence::Worse;
    }
  } else if (SCS1.Second == ICK_Pointer_Conversion &&
             SCS2.Second == ICK_Pointer_Conversion) {
    const ObjCObjectPointerType *FromPtr1
      = FromType1->getAs<ObjCObjectPointerType>();
    const ObjCObjectPointerType *FromPtr2
      = FromType2->getAs<ObjCObjectPointerType>();
    const ObjCObjectPointerType *ToPtr1
      = ToType1->getAs<ObjCObjectPointerType>();
    const ObjCObjectPointerType *ToPtr2
      = ToType2->getAs<ObjCObjectPointerType>();

    if (FromPtr1 && FromPtr2 && ToPtr1 && ToPtr2) {
      // Apply the same conversion ranking rules for Objective-C pointer types
      // that we do for C++ pointers to class types. However, we employ the
      // Objective-C pseudo-subtyping relationship used for assignment of
      // Objective-C pointer types.
      bool FromAssignLeft
        = S.Context.canAssignObjCInterfaces(FromPtr1, FromPtr2);
      bool FromAssignRight
        = S.Context.canAssignObjCInterfaces(FromPtr2, FromPtr1);
      bool ToAssignLeft
        = S.Context.canAssignObjCInterfaces(ToPtr1, ToPtr2);
      bool ToAssignRight
        = S.Context.canAssignObjCInterfaces(ToPtr2, ToPtr1);

      // A conversion to an a non-id object pointer type or qualified 'id'
      // type is better than a conversion to 'id'.
      if (ToPtr1->isObjCIdType() &&
          (ToPtr2->isObjCQualifiedIdType() || ToPtr2->getInterfaceDecl()))
        return ImplicitConversionSequence::Worse;
      if (ToPtr2->isObjCIdType() &&
          (ToPtr1->isObjCQualifiedIdType() || ToPtr1->getInterfaceDecl()))
        return ImplicitConversionSequence::Better;

      // A conversion to a non-id object pointer type is better than a
      // conversion to a qualified 'id' type
      if (ToPtr1->isObjCQualifiedIdType() && ToPtr2->getInterfaceDecl())
        return ImplicitConversionSequence::Worse;
      if (ToPtr2->isObjCQualifiedIdType() && ToPtr1->getInterfaceDecl())
        return ImplicitConversionSequence::Better;

      // A conversion to an a non-Class object pointer type or qualified 'Class'
      // type is better than a conversion to 'Class'.
      if (ToPtr1->isObjCClassType() &&
          (ToPtr2->isObjCQualifiedClassType() || ToPtr2->getInterfaceDecl()))
        return ImplicitConversionSequence::Worse;
      if (ToPtr2->isObjCClassType() &&
          (ToPtr1->isObjCQualifiedClassType() || ToPtr1->getInterfaceDecl()))
        return ImplicitConversionSequence::Better;

      // A conversion to a non-Class object pointer type is better than a
      // conversion to a qualified 'Class' type.
      if (ToPtr1->isObjCQualifiedClassType() && ToPtr2->getInterfaceDecl())
        return ImplicitConversionSequence::Worse;
      if (ToPtr2->isObjCQualifiedClassType() && ToPtr1->getInterfaceDecl())
        return ImplicitConversionSequence::Better;

      //   -- "conversion of C* to B* is better than conversion of C* to A*,"
      if (S.Context.hasSameType(FromType1, FromType2) &&
          !FromPtr1->isObjCIdType() && !FromPtr1->isObjCClassType() &&
          (ToAssignLeft != ToAssignRight)) {
        if (FromPtr1->isSpecialized()) {
          // "conversion of B<A> * to B * is better than conversion of B * to
          // C *.
          bool IsFirstSame =
              FromPtr1->getInterfaceDecl() == ToPtr1->getInterfaceDecl();
          bool IsSecondSame =
              FromPtr1->getInterfaceDecl() == ToPtr2->getInterfaceDecl();
          if (IsFirstSame) {
            if (!IsSecondSame)
              return ImplicitConversionSequence::Better;
          } else if (IsSecondSame)
            return ImplicitConversionSequence::Worse;
        }
        return ToAssignLeft? ImplicitConversionSequence::Worse
                           : ImplicitConversionSequence::Better;
      }

      //   -- "conversion of B* to A* is better than conversion of C* to A*,"
      if (S.Context.hasSameUnqualifiedType(ToType1, ToType2) &&
          (FromAssignLeft != FromAssignRight))
        return FromAssignLeft? ImplicitConversionSequence::Better
        : ImplicitConversionSequence::Worse;
    }
  }

  // Ranking of member-pointer types.
  if (SCS1.Second == ICK_Pointer_Member && SCS2.Second == ICK_Pointer_Member &&
      FromType1->isMemberPointerType() && FromType2->isMemberPointerType() &&
      ToType1->isMemberPointerType() && ToType2->isMemberPointerType()) {
    const auto *FromMemPointer1 = FromType1->castAs<MemberPointerType>();
    const auto *ToMemPointer1 = ToType1->castAs<MemberPointerType>();
    const auto *FromMemPointer2 = FromType2->castAs<MemberPointerType>();
    const auto *ToMemPointer2 = ToType2->castAs<MemberPointerType>();
    const Type *FromPointeeType1 = FromMemPointer1->getClass();
    const Type *ToPointeeType1 = ToMemPointer1->getClass();
    const Type *FromPointeeType2 = FromMemPointer2->getClass();
    const Type *ToPointeeType2 = ToMemPointer2->getClass();
    QualType FromPointee1 = QualType(FromPointeeType1, 0).getUnqualifiedType();
    QualType ToPointee1 = QualType(ToPointeeType1, 0).getUnqualifiedType();
    QualType FromPointee2 = QualType(FromPointeeType2, 0).getUnqualifiedType();
    QualType ToPointee2 = QualType(ToPointeeType2, 0).getUnqualifiedType();
    // conversion of A::* to B::* is better than conversion of A::* to C::*,
    if (FromPointee1 == FromPointee2 && ToPointee1 != ToPointee2) {
      if (S.IsDerivedFrom(Loc, ToPointee1, ToPointee2))
        return ImplicitConversionSequence::Worse;
      else if (S.IsDerivedFrom(Loc, ToPointee2, ToPointee1))
        return ImplicitConversionSequence::Better;
    }
    // conversion of B::* to C::* is better than conversion of A::* to C::*
    if (ToPointee1 == ToPointee2 && FromPointee1 != FromPointee2) {
      if (S.IsDerivedFrom(Loc, FromPointee1, FromPointee2))
        return ImplicitConversionSequence::Better;
      else if (S.IsDerivedFrom(Loc, FromPointee2, FromPointee1))
        return ImplicitConversionSequence::Worse;
    }
  }

  if (SCS1.Second == ICK_Derived_To_Base) {
    //   -- conversion of C to B is better than conversion of C to A,
    //   -- binding of an expression of type C to a reference of type
    //      B& is better than binding an expression of type C to a
    //      reference of type A&,
    if (S.Context.hasSameUnqualifiedType(FromType1, FromType2) &&
        !S.Context.hasSameUnqualifiedType(ToType1, ToType2)) {
      if (S.IsDerivedFrom(Loc, ToType1, ToType2))
        return ImplicitConversionSequence::Better;
      else if (S.IsDerivedFrom(Loc, ToType2, ToType1))
        return ImplicitConversionSequence::Worse;
    }

    //   -- conversion of B to A is better than conversion of C to A.
    //   -- binding of an expression of type B to a reference of type
    //      A& is better than binding an expression of type C to a
    //      reference of type A&,
    if (!S.Context.hasSameUnqualifiedType(FromType1, FromType2) &&
        S.Context.hasSameUnqualifiedType(ToType1, ToType2)) {
      if (S.IsDerivedFrom(Loc, FromType2, FromType1))
        return ImplicitConversionSequence::Better;
      else if (S.IsDerivedFrom(Loc, FromType1, FromType2))
        return ImplicitConversionSequence::Worse;
    }
  }

  return ImplicitConversionSequence::Indistinguishable;
}

static QualType withoutUnaligned(ASTContext &Ctx, QualType T) {
  if (!T.getQualifiers().hasUnaligned())
    return T;

  Qualifiers Q;
  T = Ctx.getUnqualifiedArrayType(T, Q);
  Q.removeUnaligned();
  return Ctx.getQualifiedType(T, Q);
}

/// CompareReferenceRelationship - Compare the two types T1 and T2 to
/// determine whether they are reference-compatible,
/// reference-related, or incompatible, for use in C++ initialization by
/// reference (C++ [dcl.ref.init]p4). Neither type can be a reference
/// type, and the first type (T1) is the pointee type of the reference
/// type being initialized.
Sema::ReferenceCompareResult
Sema::CompareReferenceRelationship(SourceLocation Loc,
                                   QualType OrigT1, QualType OrigT2,
                                   ReferenceConversions *ConvOut) {
  assert(!OrigT1->isReferenceType() &&
    "T1 must be the pointee type of the reference type");
  assert(!OrigT2->isReferenceType() && "T2 cannot be a reference type");

  QualType T1 = Context.getCanonicalType(OrigT1);
  QualType T2 = Context.getCanonicalType(OrigT2);
  Qualifiers T1Quals, T2Quals;
  QualType UnqualT1 = Context.getUnqualifiedArrayType(T1, T1Quals);
  QualType UnqualT2 = Context.getUnqualifiedArrayType(T2, T2Quals);

  ReferenceConversions ConvTmp;
  ReferenceConversions &Conv = ConvOut ? *ConvOut : ConvTmp;
  Conv = ReferenceConversions();

  // C++2a [dcl.init.ref]p4:
  //   Given types "cv1 T1" and "cv2 T2," "cv1 T1" is
  //   reference-related to "cv2 T2" if T1 is similar to T2, or
  //   T1 is a base class of T2.
  //   "cv1 T1" is reference-compatible with "cv2 T2" if
  //   a prvalue of type "pointer to cv2 T2" can be converted to the type
  //   "pointer to cv1 T1" via a standard conversion sequence.

  // Check for standard conversions we can apply to pointers: derived-to-base
  // conversions, ObjC pointer conversions, and function pointer conversions.
  // (Qualification conversions are checked last.)
  QualType ConvertedT2;
  if (UnqualT1 == UnqualT2) {
    // Nothing to do.
  } else if (isCompleteType(Loc, OrigT2) &&
             IsDerivedFrom(Loc, UnqualT2, UnqualT1))
    Conv |= ReferenceConversions::DerivedToBase;
  else if (UnqualT1->isObjCObjectOrInterfaceType() &&
           UnqualT2->isObjCObjectOrInterfaceType() &&
           Context.canBindObjCObjectType(UnqualT1, UnqualT2))
    Conv |= ReferenceConversions::ObjC;
  else if (UnqualT2->isFunctionType() &&
           IsFunctionConversion(UnqualT2, UnqualT1, ConvertedT2)) {
    Conv |= ReferenceConversions::Function;
    // No need to check qualifiers; function types don't have them.
    return Ref_Compatible;
  }
  bool ConvertedReferent = Conv != 0;

  // We can have a qualification conversion. Compute whether the types are
  // similar at the same time.
  bool PreviousToQualsIncludeConst = true;
  bool TopLevel = true;
  do {
    if (T1 == T2)
      break;

    // We will need a qualification conversion.
    Conv |= ReferenceConversions::Qualification;

    // Track whether we performed a qualification conversion anywhere other
    // than the top level. This matters for ranking reference bindings in
    // overload resolution.
    if (!TopLevel)
      Conv |= ReferenceConversions::NestedQualification;

    // MS compiler ignores __unaligned qualifier for references; do the same.
    T1 = withoutUnaligned(Context, T1);
    T2 = withoutUnaligned(Context, T2);

    // If we find a qualifier mismatch, the types are not reference-compatible,
    // but are still be reference-related if they're similar.
    bool ObjCLifetimeConversion = false;
    if (!isQualificationConversionStep(T2, T1, /*CStyle=*/false, TopLevel,
                                       PreviousToQualsIncludeConst,
                                       ObjCLifetimeConversion))
      return (ConvertedReferent || Context.hasSimilarType(T1, T2))
                 ? Ref_Related
                 : Ref_Incompatible;

    // FIXME: Should we track this for any level other than the first?
    if (ObjCLifetimeConversion)
      Conv |= ReferenceConversions::ObjCLifetime;

    TopLevel = false;
  } while (Context.UnwrapSimilarTypes(T1, T2));

  // At this point, if the types are reference-related, we must either have the
  // same inner type (ignoring qualifiers), or must have already worked out how
  // to convert the referent.
  return (ConvertedReferent || Context.hasSameUnqualifiedType(T1, T2))
             ? Ref_Compatible
             : Ref_Incompatible;
}

/// Look for a user-defined conversion to a value reference-compatible
///        with DeclType. Return true if something definite is found.
static bool
FindConversionForRefInit(Sema &S, ImplicitConversionSequence &ICS,
                         QualType DeclType, SourceLocation DeclLoc,
                         Expr *Init, QualType T2, bool AllowRvalues,
                         bool AllowExplicit) {
  assert(T2->isRecordType() && "Can only find conversions of record types.");
  auto *T2RecordDecl = cast<CXXRecordDecl>(T2->castAs<RecordType>()->getDecl());

  OverloadCandidateSet CandidateSet(
      DeclLoc, OverloadCandidateSet::CSK_InitByUserDefinedConversion);
  const auto &Conversions = T2RecordDecl->getVisibleConversionFunctions();
  for (auto I = Conversions.begin(), E = Conversions.end(); I != E; ++I) {
    NamedDecl *D = *I;
    CXXRecordDecl *ActingDC = cast<CXXRecordDecl>(D->getDeclContext());
    if (isa<UsingShadowDecl>(D))
      D = cast<UsingShadowDecl>(D)->getTargetDecl();

    FunctionTemplateDecl *ConvTemplate
      = dyn_cast<FunctionTemplateDecl>(D);
    CXXConversionDecl *Conv;
    if (ConvTemplate)
      Conv = cast<CXXConversionDecl>(ConvTemplate->getTemplatedDecl());
    else
      Conv = cast<CXXConversionDecl>(D);

    if (AllowRvalues) {
      // If we are initializing an rvalue reference, don't permit conversion
      // functions that return lvalues.
      if (!ConvTemplate && DeclType->isRValueReferenceType()) {
        const ReferenceType *RefType
          = Conv->getConversionType()->getAs<LValueReferenceType>();
        if (RefType && !RefType->getPointeeType()->isFunctionType())
          continue;
      }

      if (!ConvTemplate &&
          S.CompareReferenceRelationship(
              DeclLoc,
              Conv->getConversionType()
                  .getNonReferenceType()
                  .getUnqualifiedType(),
              DeclType.getNonReferenceType().getUnqualifiedType()) ==
              Sema::Ref_Incompatible)
        continue;
    } else {
      // If the conversion function doesn't return a reference type,
      // it can't be considered for this conversion. An rvalue reference
      // is only acceptable if its referencee is a function type.

      const ReferenceType *RefType =
        Conv->getConversionType()->getAs<ReferenceType>();
      if (!RefType ||
          (!RefType->isLValueReferenceType() &&
           !RefType->getPointeeType()->isFunctionType()))
        continue;
    }

    if (ConvTemplate)
      S.AddTemplateConversionCandidate(
          ConvTemplate, I.getPair(), ActingDC, Init, DeclType, CandidateSet,
          /*AllowObjCConversionOnExplicit=*/false, AllowExplicit);
    else
      S.AddConversionCandidate(
          Conv, I.getPair(), ActingDC, Init, DeclType, CandidateSet,
          /*AllowObjCConversionOnExplicit=*/false, AllowExplicit);
  }

  bool HadMultipleCandidates = (CandidateSet.size() > 1);

  OverloadCandidateSet::iterator Best;
  switch (CandidateSet.BestViableFunction(S, DeclLoc, Best)) {
  case OR_Success:
    // C++ [over.ics.ref]p1:
    //
    //   [...] If the parameter binds directly to the result of
    //   applying a conversion function to the argument
    //   expression, the implicit conversion sequence is a
    //   user-defined conversion sequence (13.3.3.1.2), with the
    //   second standard conversion sequence either an identity
    //   conversion or, if the conversion function returns an
    //   entity of a type that is a derived class of the parameter
    //   type, a derived-to-base Conversion.
    if (!Best->FinalConversion.DirectBinding)
      return false;

    ICS.setUserDefined();
    ICS.UserDefined.Before = Best->Conversions[0].Standard;
    ICS.UserDefined.After = Best->FinalConversion;
    ICS.UserDefined.HadMultipleCandidates = HadMultipleCandidates;
    ICS.UserDefined.ConversionFunction = Best->Function;
    ICS.UserDefined.FoundConversionFunction = Best->FoundDecl;
    ICS.UserDefined.EllipsisConversion = false;
    assert(ICS.UserDefined.After.ReferenceBinding &&
           ICS.UserDefined.After.DirectBinding &&
           "Expected a direct reference binding!");
    return true;

  case OR_Ambiguous:
    ICS.setAmbiguous();
    for (OverloadCandidateSet::iterator Cand = CandidateSet.begin();
         Cand != CandidateSet.end(); ++Cand)
      if (Cand->Best)
        ICS.Ambiguous.addConversion(Cand->FoundDecl, Cand->Function);
    return true;

  case OR_No_Viable_Function:
  case OR_Deleted:
    // There was no suitable conversion, or we found a deleted
    // conversion; continue with other checks.
    return false;
  }

  llvm_unreachable("Invalid OverloadResult!");
}

/// Compute an implicit conversion sequence for reference
/// initialization.
static ImplicitConversionSequence
TryReferenceInit(Sema &S, Expr *Init, QualType DeclType,
                 SourceLocation DeclLoc,
                 bool SuppressUserConversions,
                 bool AllowExplicit) {
  assert(DeclType->isReferenceType() && "Reference init needs a reference");

  // Most paths end in a failed conversion.
  ImplicitConversionSequence ICS;
  ICS.setBad(BadConversionSequence::no_conversion, Init, DeclType);

  QualType T1 = DeclType->castAs<ReferenceType>()->getPointeeType();
  QualType T2 = Init->getType();

  // If the initializer is the address of an overloaded function, try
  // to resolve the overloaded function. If all goes well, T2 is the
  // type of the resulting function.
  if (S.Context.getCanonicalType(T2) == S.Context.OverloadTy) {
    DeclAccessPair Found;
    if (FunctionDecl *Fn = S.ResolveAddressOfOverloadedFunction(Init, DeclType,
                                                                false, Found))
      T2 = Fn->getType();
  }

  // Compute some basic properties of the types and the initializer.
  bool isRValRef = DeclType->isRValueReferenceType();
  Expr::Classification InitCategory = Init->Classify(S.Context);

  Sema::ReferenceConversions RefConv;
  Sema::ReferenceCompareResult RefRelationship =
      S.CompareReferenceRelationship(DeclLoc, T1, T2, &RefConv);

  auto SetAsReferenceBinding = [&](bool BindsDirectly) {
    ICS.setStandard();
    ICS.Standard.First = ICK_Identity;
    // FIXME: A reference binding can be a function conversion too. We should
    // consider that when ordering reference-to-function bindings.
    ICS.Standard.Second = (RefConv & Sema::ReferenceConversions::DerivedToBase)
                              ? ICK_Derived_To_Base
                              : (RefConv & Sema::ReferenceConversions::ObjC)
                                    ? ICK_Compatible_Conversion
                                    : ICK_Identity;
    // FIXME: As a speculative fix to a defect introduced by CWG2352, we rank
    // a reference binding that performs a non-top-level qualification
    // conversion as a qualification conversion, not as an identity conversion.
    ICS.Standard.Third = (RefConv &
                              Sema::ReferenceConversions::NestedQualification)
                             ? ICK_Qualification
                             : ICK_Identity;
    ICS.Standard.setFromType(T2);
    ICS.Standard.setToType(0, T2);
    ICS.Standard.setToType(1, T1);
    ICS.Standard.setToType(2, T1);
    ICS.Standard.ReferenceBinding = true;
    ICS.Standard.DirectBinding = BindsDirectly;
    ICS.Standard.IsLvalueReference = !isRValRef;
    ICS.Standard.BindsToFunctionLvalue = T2->isFunctionType();
    ICS.Standard.BindsToRvalue = InitCategory.isRValue();
    ICS.Standard.BindsImplicitObjectArgumentWithoutRefQualifier = false;
    ICS.Standard.ObjCLifetimeConversionBinding =
        (RefConv & Sema::ReferenceConversions::ObjCLifetime) != 0;
    ICS.Standard.CopyConstructor = nullptr;
    ICS.Standard.DeprecatedStringLiteralToCharPtr = false;
  };

  // C++0x [dcl.init.ref]p5:
  //   A reference to type "cv1 T1" is initialized by an expression
  //   of type "cv2 T2" as follows:

  //     -- If reference is an lvalue reference and the initializer expression
  if (!isRValRef) {
    //     -- is an lvalue (but is not a bit-field), and "cv1 T1" is
    //        reference-compatible with "cv2 T2," or
    //
    // Per C++ [over.ics.ref]p4, we don't check the bit-field property here.
    if (InitCategory.isLValue() && RefRelationship == Sema::Ref_Compatible) {
      // C++ [over.ics.ref]p1:
      //   When a parameter of reference type binds directly (8.5.3)
      //   to an argument expression, the implicit conversion sequence
      //   is the identity conversion, unless the argument expression
      //   has a type that is a derived class of the parameter type,
      //   in which case the implicit conversion sequence is a
      //   derived-to-base Conversion (13.3.3.1).
      SetAsReferenceBinding(/*BindsDirectly=*/true);

      // Nothing more to do: the inaccessibility/ambiguity check for
      // derived-to-base conversions is suppressed when we're
      // computing the implicit conversion sequence (C++
      // [over.best.ics]p2).
      return ICS;
    }

    //       -- has a class type (i.e., T2 is a class type), where T1 is
    //          not reference-related to T2, and can be implicitly
    //          converted to an lvalue of type "cv3 T3," where "cv1 T1"
    //          is reference-compatible with "cv3 T3" 92) (this
    //          conversion is selected by enumerating the applicable
    //          conversion functions (13.3.1.6) and choosing the best
    //          one through overload resolution (13.3)),
    if (!SuppressUserConversions && T2->isRecordType() &&
        S.isCompleteType(DeclLoc, T2) &&
        RefRelationship == Sema::Ref_Incompatible) {
      if (FindConversionForRefInit(S, ICS, DeclType, DeclLoc,
                                   Init, T2, /*AllowRvalues=*/false,
                                   AllowExplicit))
        return ICS;
    }
  }

  //     -- Otherwise, the reference shall be an lvalue reference to a
  //        non-volatile const type (i.e., cv1 shall be const), or the reference
  //        shall be an rvalue reference.
  if (!isRValRef && (!T1.isConstQualified() || T1.isVolatileQualified())) {
    if (InitCategory.isRValue() && RefRelationship != Sema::Ref_Incompatible)
      ICS.setBad(BadConversionSequence::lvalue_ref_to_rvalue, Init, DeclType);
    return ICS;
  }

  //       -- If the initializer expression
  //
  //            -- is an xvalue, class prvalue, array prvalue or function
  //               lvalue and "cv1 T1" is reference-compatible with "cv2 T2", or
  if (RefRelationship == Sema::Ref_Compatible &&
      (InitCategory.isXValue() ||
       (InitCategory.isPRValue() &&
          (T2->isRecordType() || T2->isArrayType())) ||
       (InitCategory.isLValue() && T2->isFunctionType()))) {
    // In C++11, this is always a direct binding. In C++98/03, it's a direct
    // binding unless we're binding to a class prvalue.
    // Note: Although xvalues wouldn't normally show up in C++98/03 code, we
    // allow the use of rvalue references in C++98/03 for the benefit of
    // standard library implementors; therefore, we need the xvalue check here.
    SetAsReferenceBinding(/*BindsDirectly=*/S.getLangOpts().CPlusPlus11 ||
                          !(InitCategory.isPRValue() || T2->isRecordType()));
    return ICS;
  }

  //            -- has a class type (i.e., T2 is a class type), where T1 is not
  //               reference-related to T2, and can be implicitly converted to
  //               an xvalue, class prvalue, or function lvalue of type
  //               "cv3 T3", where "cv1 T1" is reference-compatible with
  //               "cv3 T3",
  //
  //          then the reference is bound to the value of the initializer
  //          expression in the first case and to the result of the conversion
  //          in the second case (or, in either case, to an appropriate base
  //          class subobject).
  if (!SuppressUserConversions && RefRelationship == Sema::Ref_Incompatible &&
      T2->isRecordType() && S.isCompleteType(DeclLoc, T2) &&
      FindConversionForRefInit(S, ICS, DeclType, DeclLoc,
                               Init, T2, /*AllowRvalues=*/true,
                               AllowExplicit)) {
    // In the second case, if the reference is an rvalue reference
    // and the second standard conversion sequence of the
    // user-defined conversion sequence includes an lvalue-to-rvalue
    // conversion, the program is ill-formed.
    if (ICS.isUserDefined() && isRValRef &&
        ICS.UserDefined.After.First == ICK_Lvalue_To_Rvalue)
      ICS.setBad(BadConversionSequence::no_conversion, Init, DeclType);

    return ICS;
  }

  // A temporary of function type cannot be created; don't even try.
  if (T1->isFunctionType())
    return ICS;

  //       -- Otherwise, a temporary of type "cv1 T1" is created and
  //          initialized from the initializer expression using the
  //          rules for a non-reference copy initialization (8.5). The
  //          reference is then bound to the temporary. If T1 is
  //          reference-related to T2, cv1 must be the same
  //          cv-qualification as, or greater cv-qualification than,
  //          cv2; otherwise, the program is ill-formed.
  if (RefRelationship == Sema::Ref_Related) {
    // If cv1 == cv2 or cv1 is a greater cv-qualified than cv2, then
    // we would be reference-compatible or reference-compatible with
    // added qualification. But that wasn't the case, so the reference
    // initialization fails.
    //
    // Note that we only want to check address spaces and cvr-qualifiers here.
    // ObjC GC, lifetime and unaligned qualifiers aren't important.
    Qualifiers T1Quals = T1.getQualifiers();
    Qualifiers T2Quals = T2.getQualifiers();
    T1Quals.removeObjCGCAttr();
    T1Quals.removeObjCLifetime();
    T2Quals.removeObjCGCAttr();
    T2Quals.removeObjCLifetime();
    // MS compiler ignores __unaligned qualifier for references; do the same.
    T1Quals.removeUnaligned();
    T2Quals.removeUnaligned();
    if (!T1Quals.compatiblyIncludes(T2Quals))
      return ICS;
  }

  // If at least one of the types is a class type, the types are not
  // related, and we aren't allowed any user conversions, the
  // reference binding fails. This case is important for breaking
  // recursion, since TryImplicitConversion below will attempt to
  // create a temporary through the use of a copy constructor.
  if (SuppressUserConversions && RefRelationship == Sema::Ref_Incompatible &&
      (T1->isRecordType() || T2->isRecordType()))
    return ICS;

  // If T1 is reference-related to T2 and the reference is an rvalue
  // reference, the initializer expression shall not be an lvalue.
  if (RefRelationship >= Sema::Ref_Related && isRValRef &&
      Init->Classify(S.Context).isLValue()) {
    ICS.setBad(BadConversionSequence::rvalue_ref_to_lvalue, Init, DeclType);
    return ICS;
  }

  // C++ [over.ics.ref]p2:
  //   When a parameter of reference type is not bound directly to
  //   an argument expression, the conversion sequence is the one
  //   required to convert the argument expression to the
  //   underlying type of the reference according to
  //   13.3.3.1. Conceptually, this conversion sequence corresponds
  //   to copy-initializing a temporary of the underlying type with
  //   the argument expression. Any difference in top-level
  //   cv-qualification is subsumed by the initialization itself
  //   and does not constitute a conversion.
  ICS = TryImplicitConversion(S, Init, T1, SuppressUserConversions,
                              AllowedExplicit::None,
                              /*InOverloadResolution=*/false,
                              /*CStyle=*/false,
                              /*AllowObjCWritebackConversion=*/false,
                              /*AllowObjCConversionOnExplicit=*/false);

  // Of course, that's still a reference binding.
  if (ICS.isStandard()) {
    ICS.Standard.ReferenceBinding = true;
    ICS.Standard.IsLvalueReference = !isRValRef;
    ICS.Standard.BindsToFunctionLvalue = false;
    ICS.Standard.BindsToRvalue = true;
    ICS.Standard.BindsImplicitObjectArgumentWithoutRefQualifier = false;
    ICS.Standard.ObjCLifetimeConversionBinding = false;
  } else if (ICS.isUserDefined()) {
    const ReferenceType *LValRefType =
        ICS.UserDefined.ConversionFunction->getReturnType()
            ->getAs<LValueReferenceType>();

    // C++ [over.ics.ref]p3:
    //   Except for an implicit object parameter, for which see 13.3.1, a
    //   standard conversion sequence cannot be formed if it requires [...]
    //   binding an rvalue reference to an lvalue other than a function
    //   lvalue.
    // Note that the function case is not possible here.
    if (isRValRef && LValRefType) {
      ICS.setBad(BadConversionSequence::no_conversion, Init, DeclType);
      return ICS;
    }

    ICS.UserDefined.After.ReferenceBinding = true;
    ICS.UserDefined.After.IsLvalueReference = !isRValRef;
    ICS.UserDefined.After.BindsToFunctionLvalue = false;
    ICS.UserDefined.After.BindsToRvalue = !LValRefType;
    ICS.UserDefined.After.BindsImplicitObjectArgumentWithoutRefQualifier = false;
    ICS.UserDefined.After.ObjCLifetimeConversionBinding = false;
  }

  return ICS;
}

static ImplicitConversionSequence
TryCopyInitialization(Sema &S, Expr *From, QualType ToType,
                      bool SuppressUserConversions,
                      bool InOverloadResolution,
                      bool AllowObjCWritebackConversion,
                      bool AllowExplicit = false);

/// TryListConversion - Try to copy-initialize a value of type ToType from the
/// initializer list From.
static ImplicitConversionSequence
TryListConversion(Sema &S, InitListExpr *From, QualType ToType,
                  bool SuppressUserConversions,
                  bool InOverloadResolution,
                  bool AllowObjCWritebackConversion) {
  // C++11 [over.ics.list]p1:
  //   When an argument is an initializer list, it is not an expression and
  //   special rules apply for converting it to a parameter type.

  ImplicitConversionSequence Result;
  Result.setBad(BadConversionSequence::no_conversion, From, ToType);

  // We need a complete type for what follows.  With one C++20 exception,
  // incomplete types can never be initialized from init lists.
  QualType InitTy = ToType;
  const ArrayType *AT = S.Context.getAsArrayType(ToType);
  if (AT && S.getLangOpts().CPlusPlus20)
    if (const auto *IAT = dyn_cast<IncompleteArrayType>(AT))
      // C++20 allows list initialization of an incomplete array type.
      InitTy = IAT->getElementType();
  if (!S.isCompleteType(From->getBeginLoc(), InitTy))
    return Result;

  // C++20 [over.ics.list]/2:
  //   If the initializer list is a designated-initializer-list, a conversion
  //   is only possible if the parameter has an aggregate type
  //
  // FIXME: The exception for reference initialization here is not part of the
  // language rules, but follow other compilers in adding it as a tentative DR
  // resolution.
  bool IsDesignatedInit = From->hasDesignatedInit();
  if (!ToType->isAggregateType() && !ToType->isReferenceType() &&
      IsDesignatedInit)
    return Result;

  // Per DR1467:
  //   If the parameter type is a class X and the initializer list has a single
  //   element of type cv U, where U is X or a class derived from X, the
  //   implicit conversion sequence is the one required to convert the element
  //   to the parameter type.
  //
  //   Otherwise, if the parameter type is a character array [... ]
  //   and the initializer list has a single element that is an
  //   appropriately-typed string literal (8.5.2 [dcl.init.string]), the
  //   implicit conversion sequence is the identity conversion.
  if (From->getNumInits() == 1 && !IsDesignatedInit) {
    if (ToType->isRecordType()) {
      QualType InitType = From->getInit(0)->getType();
      if (S.Context.hasSameUnqualifiedType(InitType, ToType) ||
          S.IsDerivedFrom(From->getBeginLoc(), InitType, ToType))
        return TryCopyInitialization(S, From->getInit(0), ToType,
                                     SuppressUserConversions,
                                     InOverloadResolution,
                                     AllowObjCWritebackConversion);
    }

    if (AT && S.IsStringInit(From->getInit(0), AT)) {
      InitializedEntity Entity =
          InitializedEntity::InitializeParameter(S.Context, ToType,
                                                 /*Consumed=*/false);
      if (S.CanPerformCopyInitialization(Entity, From)) {
        Result.setStandard();
        Result.Standard.setAsIdentityConversion();
        Result.Standard.setFromType(ToType);
        Result.Standard.setAllToTypes(ToType);
        return Result;
      }
    }
  }

  // C++14 [over.ics.list]p2: Otherwise, if the parameter type [...] (below).
  // C++11 [over.ics.list]p2:
  //   If the parameter type is std::initializer_list<X> or "array of X" and
  //   all the elements can be implicitly converted to X, the implicit
  //   conversion sequence is the worst conversion necessary to convert an
  //   element of the list to X.
  //
  // C++14 [over.ics.list]p3:
  //   Otherwise, if the parameter type is "array of N X", if the initializer
  //   list has exactly N elements or if it has fewer than N elements and X is
  //   default-constructible, and if all the elements of the initializer list
  //   can be implicitly converted to X, the implicit conversion sequence is
  //   the worst conversion necessary to convert an element of the list to X.
  if ((AT || S.isStdInitializerList(ToType, &InitTy)) && !IsDesignatedInit) {
    unsigned e = From->getNumInits();
    ImplicitConversionSequence DfltElt;
    DfltElt.setBad(BadConversionSequence::no_conversion, QualType(),
                   QualType());
    QualType ContTy = ToType;
    bool IsUnbounded = false;
    if (AT) {
      InitTy = AT->getElementType();
      if (ConstantArrayType const *CT = dyn_cast<ConstantArrayType>(AT)) {
        if (CT->getSize().ult(e)) {
          // Too many inits, fatally bad
          Result.setBad(BadConversionSequence::too_many_initializers, From,
                        ToType);
          Result.setInitializerListContainerType(ContTy, IsUnbounded);
          return Result;
        }
        if (CT->getSize().ugt(e)) {
          // Need an init from empty {}, is there one?
          InitListExpr EmptyList(S.Context, From->getEndLoc(), std::nullopt,
                                 From->getEndLoc());
          EmptyList.setType(S.Context.VoidTy);
          DfltElt = TryListConversion(
              S, &EmptyList, InitTy, SuppressUserConversions,
              InOverloadResolution, AllowObjCWritebackConversion);
          if (DfltElt.isBad()) {
            // No {} init, fatally bad
            Result.setBad(BadConversionSequence::too_few_initializers, From,
                          ToType);
            Result.setInitializerListContainerType(ContTy, IsUnbounded);
            return Result;
          }
        }
      } else {
        assert(isa<IncompleteArrayType>(AT) && "Expected incomplete array");
        IsUnbounded = true;
        if (!e) {
          // Cannot convert to zero-sized.
          Result.setBad(BadConversionSequence::too_few_initializers, From,
                        ToType);
          Result.setInitializerListContainerType(ContTy, IsUnbounded);
          return Result;
        }
        llvm::APInt Size(S.Context.getTypeSize(S.Context.getSizeType()), e);
        ContTy = S.Context.getConstantArrayType(InitTy, Size, nullptr,
                                                ArrayType::Normal, 0);
      }
    }

    Result.setStandard();
    Result.Standard.setAsIdentityConversion();
    Result.Standard.setFromType(InitTy);
    Result.Standard.setAllToTypes(InitTy);
    for (unsigned i = 0; i < e; ++i) {
      Expr *Init = From->getInit(i);
      ImplicitConversionSequence ICS = TryCopyInitialization(
          S, Init, InitTy, SuppressUserConversions, InOverloadResolution,
          AllowObjCWritebackConversion);

      // Keep the worse conversion seen so far.
      // FIXME: Sequences are not totally ordered, so 'worse' can be
      // ambiguous. CWG has been informed.
      if (CompareImplicitConversionSequences(S, From->getBeginLoc(), ICS,
                                             Result) ==
          ImplicitConversionSequence::Worse) {
        Result = ICS;
        // Bail as soon as we find something unconvertible.
        if (Result.isBad()) {
          Result.setInitializerListContainerType(ContTy, IsUnbounded);
          return Result;
        }
      }
    }

    // If we needed any implicit {} initialization, compare that now.
    // over.ics.list/6 indicates we should compare that conversion.  Again CWG
    // has been informed that this might not be the best thing.
    if (!DfltElt.isBad() && CompareImplicitConversionSequences(
                                S, From->getEndLoc(), DfltElt, Result) ==
                                ImplicitConversionSequence::Worse)
      Result = DfltElt;
    // Record the type being initialized so that we may compare sequences
    Result.setInitializerListContainerType(ContTy, IsUnbounded);
    return Result;
  }

  // C++14 [over.ics.list]p4:
  // C++11 [over.ics.list]p3:
  //   Otherwise, if the parameter is a non-aggregate class X and overload
  //   resolution chooses a single best constructor [...] the implicit
  //   conversion sequence is a user-defined conversion sequence. If multiple
  //   constructors are viable but none is better than the others, the
  //   implicit conversion sequence is a user-defined conversion sequence.
  if (ToType->isRecordType() && !ToType->isAggregateType()) {
    // This function can deal with initializer lists.
    return TryUserDefinedConversion(S, From, ToType, SuppressUserConversions,
                                    AllowedExplicit::None,
                                    InOverloadResolution, /*CStyle=*/false,
                                    AllowObjCWritebackConversion,
                                    /*AllowObjCConversionOnExplicit=*/false);
  }

  // C++14 [over.ics.list]p5:
  // C++11 [over.ics.list]p4:
  //   Otherwise, if the parameter has an aggregate type which can be
  //   initialized from the initializer list [...] the implicit conversion
  //   sequence is a user-defined conversion sequence.
  if (ToType->isAggregateType()) {
    // Type is an aggregate, argument is an init list. At this point it comes
    // down to checking whether the initialization works.
    // FIXME: Find out whether this parameter is consumed or not.
    InitializedEntity Entity =
        InitializedEntity::InitializeParameter(S.Context, ToType,
                                               /*Consumed=*/false);
    if (S.CanPerformAggregateInitializationForOverloadResolution(Entity,
                                                                 From)) {
      Result.setUserDefined();
      Result.UserDefined.Before.setAsIdentityConversion();
      // Initializer lists don't have a type.
      Result.UserDefined.Before.setFromType(QualType());
      Result.UserDefined.Before.setAllToTypes(QualType());

      Result.UserDefined.After.setAsIdentityConversion();
      Result.UserDefined.After.setFromType(ToType);
      Result.UserDefined.After.setAllToTypes(ToType);
      Result.UserDefined.ConversionFunction = nullptr;
    }
    return Result;
  }

  // C++14 [over.ics.list]p6:
  // C++11 [over.ics.list]p5:
  //   Otherwise, if the parameter is a reference, see 13.3.3.1.4.
  if (ToType->isReferenceType()) {
    // The standard is notoriously unclear here, since 13.3.3.1.4 doesn't
    // mention initializer lists in any way. So we go by what list-
    // initialization would do and try to extrapolate from that.

    QualType T1 = ToType->castAs<ReferenceType>()->getPointeeType();

    // If the initializer list has a single element that is reference-related
    // to the parameter type, we initialize the reference from that.
    if (From->getNumInits() == 1 && !IsDesignatedInit) {
      Expr *Init = From->getInit(0);

      QualType T2 = Init->getType();

      // If the initializer is the address of an overloaded function, try
      // to resolve the overloaded function. If all goes well, T2 is the
      // type of the resulting function.
      if (S.Context.getCanonicalType(T2) == S.Context.OverloadTy) {
        DeclAccessPair Found;
        if (FunctionDecl *Fn = S.ResolveAddressOfOverloadedFunction(
                                   Init, ToType, false, Found))
          T2 = Fn->getType();
      }

      // Compute some basic properties of the types and the initializer.
      Sema::ReferenceCompareResult RefRelationship =
          S.CompareReferenceRelationship(From->getBeginLoc(), T1, T2);

      if (RefRelationship >= Sema::Ref_Related) {
        return TryReferenceInit(S, Init, ToType, /*FIXME*/ From->getBeginLoc(),
                                SuppressUserConversions,
                                /*AllowExplicit=*/false);
      }
    }

    // Otherwise, we bind the reference to a temporary created from the
    // initializer list.
    Result = TryListConversion(S, From, T1, SuppressUserConversions,
                               InOverloadResolution,
                               AllowObjCWritebackConversion);
    if (Result.isFailure())
      return Result;
    assert(!Result.isEllipsis() &&
           "Sub-initialization cannot result in ellipsis conversion.");

    // Can we even bind to a temporary?
    if (ToType->isRValueReferenceType() ||
        (T1.isConstQualified() && !T1.isVolatileQualified())) {
      StandardConversionSequence &SCS = Result.isStandard() ? Result.Standard :
                                            Result.UserDefined.After;
      SCS.ReferenceBinding = true;
      SCS.IsLvalueReference = ToType->isLValueReferenceType();
      SCS.BindsToRvalue = true;
      SCS.BindsToFunctionLvalue = false;
      SCS.BindsImplicitObjectArgumentWithoutRefQualifier = false;
      SCS.ObjCLifetimeConversionBinding = false;
    } else
      Result.setBad(BadConversionSequence::lvalue_ref_to_rvalue,
                    From, ToType);
    return Result;
  }

  // C++14 [over.ics.list]p7:
  // C++11 [over.ics.list]p6:
  //   Otherwise, if the parameter type is not a class:
  if (!ToType->isRecordType()) {
    //    - if the initializer list has one element that is not itself an
    //      initializer list, the implicit conversion sequence is the one
    //      required to convert the element to the parameter type.
    unsigned NumInits = From->getNumInits();
    if (NumInits == 1 && !isa<InitListExpr>(From->getInit(0)))
      Result = TryCopyInitialization(S, From->getInit(0), ToType,
                                     SuppressUserConversions,
                                     InOverloadResolution,
                                     AllowObjCWritebackConversion);
    //    - if the initializer list has no elements, the implicit conversion
    //      sequence is the identity conversion.
    else if (NumInits == 0) {
      Result.setStandard();
      Result.Standard.setAsIdentityConversion();
      Result.Standard.setFromType(ToType);
      Result.Standard.setAllToTypes(ToType);
    }
    return Result;
  }

  // C++14 [over.ics.list]p8:
  // C++11 [over.ics.list]p7:
  //   In all cases other than those enumerated above, no conversion is possible
  return Result;
}

/// TryCopyInitialization - Try to copy-initialize a value of type
/// ToType from the expression From. Return the implicit conversion
/// sequence required to pass this argument, which may be a bad
/// conversion sequence (meaning that the argument cannot be passed to
/// a parameter of this type). If @p SuppressUserConversions, then we
/// do not permit any user-defined conversion sequences.
static ImplicitConversionSequence
TryCopyInitialization(Sema &S, Expr *From, QualType ToType,
                      bool SuppressUserConversions,
                      bool InOverloadResolution,
                      bool AllowObjCWritebackConversion,
                      bool AllowExplicit) {
  if (InitListExpr *FromInitList = dyn_cast<InitListExpr>(From))
    return TryListConversion(S, FromInitList, ToType, SuppressUserConversions,
                             InOverloadResolution,AllowObjCWritebackConversion);

  if (ToType->isReferenceType())
    return TryReferenceInit(S, From, ToType,
                            /*FIXME:*/ From->getBeginLoc(),
                            SuppressUserConversions, AllowExplicit);

  return TryImplicitConversion(S, From, ToType,
                               SuppressUserConversions,
                               AllowedExplicit::None,
                               InOverloadResolution,
                               /*CStyle=*/false,
                               AllowObjCWritebackConversion,
                               /*AllowObjCConversionOnExplicit=*/false);
}

static bool TryCopyInitialization(const CanQualType FromQTy,
                                  const CanQualType ToQTy,
                                  Sema &S,
                                  SourceLocation Loc,
                                  ExprValueKind FromVK) {
  OpaqueValueExpr TmpExpr(Loc, FromQTy, FromVK);
  ImplicitConversionSequence ICS =
    TryCopyInitialization(S, &TmpExpr, ToQTy, true, true, false);

  return !ICS.isBad();
}

/// TryObjectArgumentInitialization - Try to initialize the object
/// parameter of the given member function (@c Method) from the
/// expression @p From.
static ImplicitConversionSequence
TryObjectArgumentInitialization(Sema &S, SourceLocation Loc, QualType FromType,
                                Expr::Classification FromClassification,
                                CXXMethodDecl *Method,
                                CXXRecordDecl *ActingContext) {
  QualType ClassType = S.Context.getTypeDeclType(ActingContext);
  // [class.dtor]p2: A destructor can be invoked for a const, volatile or
  //                 const volatile object.
  Qualifiers Quals = Method->getMethodQualifiers();
  if (isa<CXXDestructorDecl>(Method)) {
    Quals.addConst();
    Quals.addVolatile();
  }

  QualType ImplicitParamType = S.Context.getQualifiedType(ClassType, Quals);

  // Set up the conversion sequence as a "bad" conversion, to allow us
  // to exit early.
  ImplicitConversionSequence ICS;

  // We need to have an object of class type.
  if (const PointerType *PT = FromType->getAs<PointerType>()) {
    FromType = PT->getPointeeType();

    // When we had a pointer, it's implicitly dereferenced, so we
    // better have an lvalue.
    assert(FromClassification.isLValue());
  }

  assert(FromType->isRecordType());

  // C++0x [over.match.funcs]p4:
  //   For non-static member functions, the type of the implicit object
  //   parameter is
  //
  //     - "lvalue reference to cv X" for functions declared without a
  //        ref-qualifier or with the & ref-qualifier
  //     - "rvalue reference to cv X" for functions declared with the &&
  //        ref-qualifier
  //
  // where X is the class of which the function is a member and cv is the
  // cv-qualification on the member function declaration.
  //
  // However, when finding an implicit conversion sequence for the argument, we
  // are not allowed to perform user-defined conversions
  // (C++ [over.match.funcs]p5). We perform a simplified version of
  // reference binding here, that allows class rvalues to bind to
  // non-constant references.

  // First check the qualifiers.
  QualType FromTypeCanon = S.Context.getCanonicalType(FromType);
  if (ImplicitParamType.getCVRQualifiers()
                                    != FromTypeCanon.getLocalCVRQualifiers() &&
      !ImplicitParamType.isAtLeastAsQualifiedAs(FromTypeCanon)) {
    ICS.setBad(BadConversionSequence::bad_qualifiers,
               FromType, ImplicitParamType);
    return ICS;
  }

  if (FromTypeCanon.hasAddressSpace()) {
    Qualifiers QualsImplicitParamType = ImplicitParamType.getQualifiers();
    Qualifiers QualsFromType = FromTypeCanon.getQualifiers();
    if (!QualsImplicitParamType.isAddressSpaceSupersetOf(QualsFromType)) {
      ICS.setBad(BadConversionSequence::bad_qualifiers,
                 FromType, ImplicitParamType);
      return ICS;
    }
  }

  // Check that we have either the same type or a derived type. It
  // affects the conversion rank.
  QualType ClassTypeCanon = S.Context.getCanonicalType(ClassType);
  ImplicitConversionKind SecondKind;
  if (ClassTypeCanon == FromTypeCanon.getLocalUnqualifiedType()) {
    SecondKind = ICK_Identity;
  } else if (S.IsDerivedFrom(Loc, FromType, ClassType))
    SecondKind = ICK_Derived_To_Base;
  else {
    ICS.setBad(BadConversionSequence::unrelated_class,
               FromType, ImplicitParamType);
    return ICS;
  }

  // Check the ref-qualifier.
  switch (Method->getRefQualifier()) {
  case RQ_None:
    // Do nothing; we don't care about lvalueness or rvalueness.
    break;

  case RQ_LValue:
    if (!FromClassification.isLValue() && !Quals.hasOnlyConst()) {
      // non-const lvalue reference cannot bind to an rvalue
      ICS.setBad(BadConversionSequence::lvalue_ref_to_rvalue, FromType,
                 ImplicitParamType);
      return ICS;
    }
    break;

  case RQ_RValue:
    if (!FromClassification.isRValue()) {
      // rvalue reference cannot bind to an lvalue
      ICS.setBad(BadConversionSequence::rvalue_ref_to_lvalue, FromType,
                 ImplicitParamType);
      return ICS;
    }
    break;
  }

  // Success. Mark this as a reference binding.
  ICS.setStandard();
  ICS.Standard.setAsIdentityConversion();
  ICS.Standard.Second = SecondKind;
  ICS.Standard.setFromType(FromType);
  ICS.Standard.setAllToTypes(ImplicitParamType);
  ICS.Standard.ReferenceBinding = true;
  ICS.Standard.DirectBinding = true;
  ICS.Standard.IsLvalueReference = Method->getRefQualifier() != RQ_RValue;
  ICS.Standard.BindsToFunctionLvalue = false;
  ICS.Standard.BindsToRvalue = FromClassification.isRValue();
  ICS.Standard.BindsImplicitObjectArgumentWithoutRefQualifier
    = (Method->getRefQualifier() == RQ_None);
  return ICS;
}

/// PerformObjectArgumentInitialization - Perform initialization of
/// the implicit object parameter for the given Method with the given
/// expression.
ExprResult
Sema::PerformObjectArgumentInitialization(Expr *From,
                                          NestedNameSpecifier *Qualifier,
                                          NamedDecl *FoundDecl,
                                          CXXMethodDecl *Method) {
  QualType FromRecordType, DestType;
  QualType ImplicitParamRecordType  =
    Method->getThisType()->castAs<PointerType>()->getPointeeType();

  Expr::Classification FromClassification;
  if (const PointerType *PT = From->getType()->getAs<PointerType>()) {
    FromRecordType = PT->getPointeeType();
    DestType = Method->getThisType();
    FromClassification = Expr::Classification::makeSimpleLValue();
  } else {
    FromRecordType = From->getType();
    DestType = ImplicitParamRecordType;
    FromClassification = From->Classify(Context);

    // When performing member access on a prvalue, materialize a temporary.
    if (From->isPRValue()) {
      From = CreateMaterializeTemporaryExpr(FromRecordType, From,
                                            Method->getRefQualifier() !=
                                                RefQualifierKind::RQ_RValue);
    }
  }

  // Note that we always use the true parent context when performing
  // the actual argument initialization.
  ImplicitConversionSequence ICS = TryObjectArgumentInitialization(
      *this, From->getBeginLoc(), From->getType(), FromClassification, Method,
      Method->getParent());
  if (ICS.isBad()) {
    switch (ICS.Bad.Kind) {
    case BadConversionSequence::bad_qualifiers: {
      Qualifiers FromQs = FromRecordType.getQualifiers();
      Qualifiers ToQs = DestType.getQualifiers();
      unsigned CVR = FromQs.getCVRQualifiers() & ~ToQs.getCVRQualifiers();
      if (CVR) {
        Diag(From->getBeginLoc(), diag::err_member_function_call_bad_cvr)
            << Method->getDeclName() << FromRecordType << (CVR - 1)
            << From->getSourceRange();
        Diag(Method->getLocation(), diag::note_previous_decl)
          << Method->getDeclName();
        return ExprError();
      }
      break;
    }

    case BadConversionSequence::lvalue_ref_to_rvalue:
    case BadConversionSequence::rvalue_ref_to_lvalue: {
      bool IsRValueQualified =
        Method->getRefQualifier() == RefQualifierKind::RQ_RValue;
      Diag(From->getBeginLoc(), diag::err_member_function_call_bad_ref)
          << Method->getDeclName() << FromClassification.isRValue()
          << IsRValueQualified;
      Diag(Method->getLocation(), diag::note_previous_decl)
        << Method->getDeclName();
      return ExprError();
    }

    case BadConversionSequence::no_conversion:
    case BadConversionSequence::unrelated_class:
      break;

    case BadConversionSequence::too_few_initializers:
    case BadConversionSequence::too_many_initializers:
      llvm_unreachable("Lists are not objects");
    }

    return Diag(From->getBeginLoc(), diag::err_member_function_call_bad_type)
           << ImplicitParamRecordType << FromRecordType
           << From->getSourceRange();
  }

  if (ICS.Standard.Second == ICK_Derived_To_Base) {
    ExprResult FromRes =
      PerformObjectMemberConversion(From, Qualifier, FoundDecl, Method);
    if (FromRes.isInvalid())
      return ExprError();
    From = FromRes.get();
  }

  if (!Context.hasSameType(From->getType(), DestType)) {
    CastKind CK;
    QualType PteeTy = DestType->getPointeeType();
    LangAS DestAS =
        PteeTy.isNull() ? DestType.getAddressSpace() : PteeTy.getAddressSpace();
    if (FromRecordType.getAddressSpace() != DestAS)
      CK = CK_AddressSpaceConversion;
    else
      CK = CK_NoOp;
    From = ImpCastExprToType(From, DestType, CK, From->getValueKind()).get();
  }
  return From;
}

/// TryContextuallyConvertToBool - Attempt to contextually convert the
/// expression From to bool (C++0x [conv]p3).
static ImplicitConversionSequence
TryContextuallyConvertToBool(Sema &S, Expr *From) {
  // C++ [dcl.init]/17.8:
  //   - Otherwise, if the initialization is direct-initialization, the source
  //     type is std::nullptr_t, and the destination type is bool, the initial
  //     value of the object being initialized is false.
  if (From->getType()->isNullPtrType())
    return ImplicitConversionSequence::getNullptrToBool(From->getType(),
                                                        S.Context.BoolTy,
                                                        From->isGLValue());

  // All other direct-initialization of bool is equivalent to an implicit
  // conversion to bool in which explicit conversions are permitted.
  return TryImplicitConversion(S, From, S.Context.BoolTy,
                               /*SuppressUserConversions=*/false,
                               AllowedExplicit::Conversions,
                               /*InOverloadResolution=*/false,
                               /*CStyle=*/false,
                               /*AllowObjCWritebackConversion=*/false,
                               /*AllowObjCConversionOnExplicit=*/false);
}

/// PerformContextuallyConvertToBool - Perform a contextual conversion
/// of the expression From to bool (C++0x [conv]p3).
ExprResult Sema::PerformContextuallyConvertToBool(Expr *From) {
  if (checkPlaceholderForOverload(*this, From))
    return ExprError();

  ImplicitConversionSequence ICS = TryContextuallyConvertToBool(*this, From);
  if (!ICS.isBad())
    return PerformImplicitConversion(From, Context.BoolTy, ICS, AA_Converting);

  if (!DiagnoseMultipleUserDefinedConversion(From, Context.BoolTy))
    return Diag(From->getBeginLoc(), diag::err_typecheck_bool_condition)
           << From->getType() << From->getSourceRange();
  return ExprError();
}

/// Check that the specified conversion is permitted in a converted constant
/// expression, according to C++11 [expr.const]p3. Return true if the conversion
/// is acceptable.
static bool CheckConvertedConstantConversions(Sema &S,
                                              StandardConversionSequence &SCS) {
  // Since we know that the target type is an integral or unscoped enumeration
  // type, most conversion kinds are impossible. All possible First and Third
  // conversions are fine.
  switch (SCS.Second) {
  case ICK_Identity:
  case ICK_Integral_Promotion:
  case ICK_Integral_Conversion: // Narrowing conversions are checked elsewhere.
  case ICK_Zero_Queue_Conversion:
    return true;

  case ICK_Boolean_Conversion:
    // Conversion from an integral or unscoped enumeration type to bool is
    // classified as ICK_Boolean_Conversion, but it's also arguably an integral
    // conversion, so we allow it in a converted constant expression.
    //
    // FIXME: Per core issue 1407, we should not allow this, but that breaks
    // a lot of popular code. We should at least add a warning for this
    // (non-conforming) extension.
    return SCS.getFromType()->isIntegralOrUnscopedEnumerationType() &&
           SCS.getToType(2)->isBooleanType();

  case ICK_Pointer_Conversion:
  case ICK_Pointer_Member:
    // C++1z: null pointer conversions and null member pointer conversions are
    // only permitted if the source type is std::nullptr_t.
    return SCS.getFromType()->isNullPtrType();

  case ICK_Floating_Promotion:
  case ICK_Complex_Promotion:
  case ICK_Floating_Conversion:
  case ICK_Complex_Conversion:
  case ICK_Floating_Integral:
  case ICK_Compatible_Conversion:
  case ICK_Derived_To_Base:
  case ICK_Vector_Conversion:
  case ICK_SVE_Vector_Conversion:
  case ICK_RVV_Vector_Conversion:
  case ICK_Vector_Splat:
  case ICK_Complex_Real:
  case ICK_Block_Pointer_Conversion:
  case ICK_TransparentUnionConversion:
  case ICK_Writeback_Conversion:
  case ICK_Zero_Event_Conversion:
  case ICK_C_Only_Conversion:
  case ICK_Incompatible_Pointer_Conversion:
    return false;

  case ICK_Lvalue_To_Rvalue:
  case ICK_Array_To_Pointer:
  case ICK_Function_To_Pointer:
    llvm_unreachable("found a first conversion kind in Second");

  case ICK_Function_Conversion:
  case ICK_Qualification:
    llvm_unreachable("found a third conversion kind in Second");

  case ICK_Num_Conversion_Kinds:
    break;
  }

  llvm_unreachable("unknown conversion kind");
}

/// CheckConvertedConstantExpression - Check that the expression From is a
/// converted constant expression of type T, perform the conversion and produce
/// the converted expression, per C++11 [expr.const]p3.
static ExprResult CheckConvertedConstantExpression(Sema &S, Expr *From,
                                                   QualType T, APValue &Value,
                                                   Sema::CCEKind CCE,
                                                   bool RequireInt,
                                                   NamedDecl *Dest) {
  assert(S.getLangOpts().CPlusPlus11 &&
         "converted constant expression outside C++11");

  if (checkPlaceholderForOverload(S, From))
    return ExprError();

  // C++1z [expr.const]p3:
  //  A converted constant expression of type T is an expression,
  //  implicitly converted to type T, where the converted
  //  expression is a constant expression and the implicit conversion
  //  sequence contains only [... list of conversions ...].
  ImplicitConversionSequence ICS =
      (CCE == Sema::CCEK_ExplicitBool || CCE == Sema::CCEK_Noexcept)
          ? TryContextuallyConvertToBool(S, From)
          : TryCopyInitialization(S, From, T,
                                  /*SuppressUserConversions=*/false,
                                  /*InOverloadResolution=*/false,
                                  /*AllowObjCWritebackConversion=*/false,
                                  /*AllowExplicit=*/false);
  StandardConversionSequence *SCS = nullptr;
  switch (ICS.getKind()) {
  case ImplicitConversionSequence::StandardConversion:
    SCS = &ICS.Standard;
    break;
  case ImplicitConversionSequence::UserDefinedConversion:
    if (T->isRecordType())
      SCS = &ICS.UserDefined.Before;
    else
      SCS = &ICS.UserDefined.After;
    break;
  case ImplicitConversionSequence::AmbiguousConversion:
  case ImplicitConversionSequence::BadConversion:
    if (!S.DiagnoseMultipleUserDefinedConversion(From, T))
      return S.Diag(From->getBeginLoc(),
                    diag::err_typecheck_converted_constant_expression)
             << From->getType() << From->getSourceRange() << T;
    return ExprError();

  case ImplicitConversionSequence::EllipsisConversion:
  case ImplicitConversionSequence::StaticObjectArgumentConversion:
    llvm_unreachable("bad conversion in converted constant expression");
  }

  // Check that we would only use permitted conversions.
  if (!CheckConvertedConstantConversions(S, *SCS)) {
    return S.Diag(From->getBeginLoc(),
                  diag::err_typecheck_converted_constant_expression_disallowed)
           << From->getType() << From->getSourceRange() << T;
  }
  // [...] and where the reference binding (if any) binds directly.
  if (SCS->ReferenceBinding && !SCS->DirectBinding) {
    return S.Diag(From->getBeginLoc(),
                  diag::err_typecheck_converted_constant_expression_indirect)
           << From->getType() << From->getSourceRange() << T;
  }

  // Usually we can simply apply the ImplicitConversionSequence we formed
  // earlier, but that's not guaranteed to work when initializing an object of
  // class type.
  ExprResult Result;
  if (T->isRecordType()) {
    assert(CCE == Sema::CCEK_TemplateArg &&
           "unexpected class type converted constant expr");
    Result = S.PerformCopyInitialization(
        InitializedEntity::InitializeTemplateParameter(
            T, cast<NonTypeTemplateParmDecl>(Dest)),
        SourceLocation(), From);
  } else {
    Result = S.PerformImplicitConversion(From, T, ICS, Sema::AA_Converting);
  }
  if (Result.isInvalid())
    return Result;

  // C++2a [intro.execution]p5:
  //   A full-expression is [...] a constant-expression [...]
  Result = S.ActOnFinishFullExpr(Result.get(), From->getExprLoc(),
                                 /*DiscardedValue=*/false, /*IsConstexpr=*/true,
                                 CCE == Sema::CCEKind::CCEK_TemplateArg);
  if (Result.isInvalid())
    return Result;

  // Check for a narrowing implicit conversion.
  bool ReturnPreNarrowingValue = false;
  APValue PreNarrowingValue;
  QualType PreNarrowingType;
  switch (SCS->getNarrowingKind(S.Context, Result.get(), PreNarrowingValue,
                                PreNarrowingType)) {
  case NK_Dependent_Narrowing:
    // Implicit conversion to a narrower type, but the expression is
    // value-dependent so we can't tell whether it's actually narrowing.
  case NK_Variable_Narrowing:
    // Implicit conversion to a narrower type, and the value is not a constant
    // expression. We'll diagnose this in a moment.
  case NK_Not_Narrowing:
    break;

  case NK_Constant_Narrowing:
    if (CCE == Sema::CCEK_ArrayBound &&
        PreNarrowingType->isIntegralOrEnumerationType() &&
        PreNarrowingValue.isInt()) {
      // Don't diagnose array bound narrowing here; we produce more precise
      // errors by allowing the un-narrowed value through.
      ReturnPreNarrowingValue = true;
      break;
    }
    S.Diag(From->getBeginLoc(), diag::ext_cce_narrowing)
        << CCE << /*Constant*/ 1
        << PreNarrowingValue.getAsString(S.Context, PreNarrowingType) << T;
    break;

  case NK_Type_Narrowing:
    // FIXME: It would be better to diagnose that the expression is not a
    // constant expression.
    S.Diag(From->getBeginLoc(), diag::ext_cce_narrowing)
        << CCE << /*Constant*/ 0 << From->getType() << T;
    break;
  }

  if (Result.get()->isValueDependent()) {
    Value = APValue();
    return Result;
  }

  // Check the expression is a constant expression.
  SmallVector<PartialDiagnosticAt, 8> Notes;
  Expr::EvalResult Eval;
  Eval.Diag = &Notes;

  ConstantExprKind Kind;
  if (CCE == Sema::CCEK_TemplateArg && T->isRecordType())
    Kind = ConstantExprKind::ClassTemplateArgument;
  else if (CCE == Sema::CCEK_TemplateArg)
    Kind = ConstantExprKind::NonClassTemplateArgument;
  else
    Kind = ConstantExprKind::Normal;

  if (!Result.get()->EvaluateAsConstantExpr(Eval, S.Context, Kind) ||
      (RequireInt && !Eval.Val.isInt())) {
    // The expression can't be folded, so we can't keep it at this position in
    // the AST.
    Result = ExprError();
  } else {
    Value = Eval.Val;

    if (Notes.empty()) {
      // It's a constant expression.
      Expr *E = ConstantExpr::Create(S.Context, Result.get(), Value);
      if (ReturnPreNarrowingValue)
        Value = std::move(PreNarrowingValue);
      return E;
    }
  }

  // It's not a constant expression. Produce an appropriate diagnostic.
  if (Notes.size() == 1 &&
      Notes[0].second.getDiagID() == diag::note_invalid_subexpr_in_const_expr) {
    S.Diag(Notes[0].first, diag::err_expr_not_cce) << CCE;
  } else if (!Notes.empty() && Notes[0].second.getDiagID() ==
                                   diag::note_constexpr_invalid_template_arg) {
    Notes[0].second.setDiagID(diag::err_constexpr_invalid_template_arg);
    for (unsigned I = 0; I < Notes.size(); ++I)
      S.Diag(Notes[I].first, Notes[I].second);
  } else {
    S.Diag(From->getBeginLoc(), diag::err_expr_not_cce)
        << CCE << From->getSourceRange();
    for (unsigned I = 0; I < Notes.size(); ++I)
      S.Diag(Notes[I].first, Notes[I].second);
  }
  return ExprError();
}

ExprResult Sema::CheckConvertedConstantExpression(Expr *From, QualType T,
                                                  APValue &Value, CCEKind CCE,
                                                  NamedDecl *Dest) {
  return ::CheckConvertedConstantExpression(*this, From, T, Value, CCE, false,
                                            Dest);
}

ExprResult Sema::CheckConvertedConstantExpression(Expr *From, QualType T,
                                                  llvm::APSInt &Value,
                                                  CCEKind CCE) {
  assert(T->isIntegralOrEnumerationType() && "unexpected converted const type");

  APValue V;
  auto R = ::CheckConvertedConstantExpression(*this, From, T, V, CCE, true,
                                              /*Dest=*/nullptr);
  if (!R.isInvalid() && !R.get()->isValueDependent())
    Value = V.getInt();
  return R;
}


/// dropPointerConversions - If the given standard conversion sequence
/// involves any pointer conversions, remove them.  This may change
/// the result type of the conversion sequence.
static void dropPointerConversion(StandardConversionSequence &SCS) {
  if (SCS.Second == ICK_Pointer_Conversion) {
    SCS.Second = ICK_Identity;
    SCS.Third = ICK_Identity;
    SCS.ToTypePtrs[2] = SCS.ToTypePtrs[1] = SCS.ToTypePtrs[0];
  }
}

/// TryContextuallyConvertToObjCPointer - Attempt to contextually
/// convert the expression From to an Objective-C pointer type.
static ImplicitConversionSequence
TryContextuallyConvertToObjCPointer(Sema &S, Expr *From) {
  // Do an implicit conversion to 'id'.
  QualType Ty = S.Context.getObjCIdType();
  ImplicitConversionSequence ICS
    = TryImplicitConversion(S, From, Ty,
                            // FIXME: Are these flags correct?
                            /*SuppressUserConversions=*/false,
                            AllowedExplicit::Conversions,
                            /*InOverloadResolution=*/false,
                            /*CStyle=*/false,
                            /*AllowObjCWritebackConversion=*/false,
                            /*AllowObjCConversionOnExplicit=*/true);

  // Strip off any final conversions to 'id'.
  switch (ICS.getKind()) {
  case ImplicitConversionSequence::BadConversion:
  case ImplicitConversionSequence::AmbiguousConversion:
  case ImplicitConversionSequence::EllipsisConversion:
  case ImplicitConversionSequence::StaticObjectArgumentConversion:
    break;

  case ImplicitConversionSequence::UserDefinedConversion:
    dropPointerConversion(ICS.UserDefined.After);
    break;

  case ImplicitConversionSequence::StandardConversion:
    dropPointerConversion(ICS.Standard);
    break;
  }

  return ICS;
}

/// PerformContextuallyConvertToObjCPointer - Perform a contextual
/// conversion of the expression From to an Objective-C pointer type.
/// Returns a valid but null ExprResult if no conversion sequence exists.
ExprResult Sema::PerformContextuallyConvertToObjCPointer(Expr *From) {
  if (checkPlaceholderForOverload(*this, From))
    return ExprError();

  QualType Ty = Context.getObjCIdType();
  ImplicitConversionSequence ICS =
    TryContextuallyConvertToObjCPointer(*this, From);
  if (!ICS.isBad())
    return PerformImplicitConversion(From, Ty, ICS, AA_Converting);
  return ExprResult();
}

/// Determine whether the provided type is an integral type, or an enumeration
/// type of a permitted flavor.
bool Sema::ICEConvertDiagnoser::match(QualType T) {
  return AllowScopedEnumerations ? T->isIntegralOrEnumerationType()
                                 : T->isIntegralOrUnscopedEnumerationType();
}

static ExprResult
diagnoseAmbiguousConversion(Sema &SemaRef, SourceLocation Loc, Expr *From,
                            Sema::ContextualImplicitConverter &Converter,
                            QualType T, UnresolvedSetImpl &ViableConversions) {

  if (Converter.Suppress)
    return ExprError();

  Converter.diagnoseAmbiguous(SemaRef, Loc, T) << From->getSourceRange();
  for (unsigned I = 0, N = ViableConversions.size(); I != N; ++I) {
    CXXConversionDecl *Conv =
        cast<CXXConversionDecl>(ViableConversions[I]->getUnderlyingDecl());
    QualType ConvTy = Conv->getConversionType().getNonReferenceType();
    Converter.noteAmbiguous(SemaRef, Conv, ConvTy);
  }
  return From;
}

static bool
diagnoseNoViableConversion(Sema &SemaRef, SourceLocation Loc, Expr *&From,
                           Sema::ContextualImplicitConverter &Converter,
                           QualType T, bool HadMultipleCandidates,
                           UnresolvedSetImpl &ExplicitConversions) {
  if (ExplicitConversions.size() == 1 && !Converter.Suppress) {
    DeclAccessPair Found = ExplicitConversions[0];
    CXXConversionDecl *Conversion =
        cast<CXXConversionDecl>(Found->getUnderlyingDecl());

    // The user probably meant to invoke the given explicit
    // conversion; use it.
    QualType ConvTy = Conversion->getConversionType().getNonReferenceType();
    std::string TypeStr;
    ConvTy.getAsStringInternal(TypeStr, SemaRef.getPrintingPolicy());

    Converter.diagnoseExplicitConv(SemaRef, Loc, T, ConvTy)
        << FixItHint::CreateInsertion(From->getBeginLoc(),
                                      "static_cast<" + TypeStr + ">(")
        << FixItHint::CreateInsertion(
               SemaRef.getLocForEndOfToken(From->getEndLoc()), ")");
    Converter.noteExplicitConv(SemaRef, Conversion, ConvTy);

    // If we aren't in a SFINAE context, build a call to the
    // explicit conversion function.
    if (SemaRef.isSFINAEContext())
      return true;

    SemaRef.CheckMemberOperatorAccess(From->getExprLoc(), From, nullptr, Found);
    ExprResult Result = SemaRef.BuildCXXMemberCallExpr(From, Found, Conversion,
                                                       HadMultipleCandidates);
    if (Result.isInvalid())
      return true;
    // Record usage of conversion in an implicit cast.
    From = ImplicitCastExpr::Create(SemaRef.Context, Result.get()->getType(),
                                    CK_UserDefinedConversion, Result.get(),
                                    nullptr, Result.get()->getValueKind(),
                                    SemaRef.CurFPFeatureOverrides());
  }
  return false;
}

static bool recordConversion(Sema &SemaRef, SourceLocation Loc, Expr *&From,
                             Sema::ContextualImplicitConverter &Converter,
                             QualType T, bool HadMultipleCandidates,
                             DeclAccessPair &Found) {
  CXXConversionDecl *Conversion =
      cast<CXXConversionDecl>(Found->getUnderlyingDecl());
  SemaRef.CheckMemberOperatorAccess(From->getExprLoc(), From, nullptr, Found);

  QualType ToType = Conversion->getConversionType().getNonReferenceType();
  if (!Converter.SuppressConversion) {
    if (SemaRef.isSFINAEContext())
      return true;

    Converter.diagnoseConversion(SemaRef, Loc, T, ToType)
        << From->getSourceRange();
  }

  ExprResult Result = SemaRef.BuildCXXMemberCallExpr(From, Found, Conversion,
                                                     HadMultipleCandidates);
  if (Result.isInvalid())
    return true;
  // Record usage of conversion in an implicit cast.
  From = ImplicitCastExpr::Create(SemaRef.Context, Result.get()->getType(),
                                  CK_UserDefinedConversion, Result.get(),
                                  nullptr, Result.get()->getValueKind(),
                                  SemaRef.CurFPFeatureOverrides());
  return false;
}

static ExprResult finishContextualImplicitConversion(
    Sema &SemaRef, SourceLocation Loc, Expr *From,
    Sema::ContextualImplicitConverter &Converter) {
  if (!Converter.match(From->getType()) && !Converter.Suppress)
    Converter.diagnoseNoMatch(SemaRef, Loc, From->getType())
        << From->getSourceRange();

  return SemaRef.DefaultLvalueConversion(From);
}

static void
collectViableConversionCandidates(Sema &SemaRef, Expr *From, QualType ToType,
                                  UnresolvedSetImpl &ViableConversions,
                                  OverloadCandidateSet &CandidateSet) {
  for (unsigned I = 0, N = ViableConversions.size(); I != N; ++I) {
    DeclAccessPair FoundDecl = ViableConversions[I];
    NamedDecl *D = FoundDecl.getDecl();
    CXXRecordDecl *ActingContext = cast<CXXRecordDecl>(D->getDeclContext());
    if (isa<UsingShadowDecl>(D))
      D = cast<UsingShadowDecl>(D)->getTargetDecl();

    CXXConversionDecl *Conv;
    FunctionTemplateDecl *ConvTemplate;
    if ((ConvTemplate = dyn_cast<FunctionTemplateDecl>(D)))
      Conv = cast<CXXConversionDecl>(ConvTemplate->getTemplatedDecl());
    else
      Conv = cast<CXXConversionDecl>(D);

    if (ConvTemplate)
      SemaRef.AddTemplateConversionCandidate(
          ConvTemplate, FoundDecl, ActingContext, From, ToType, CandidateSet,
          /*AllowObjCConversionOnExplicit=*/false, /*AllowExplicit*/ true);
    else
      SemaRef.AddConversionCandidate(Conv, FoundDecl, ActingContext, From,
                                     ToType, CandidateSet,
                                     /*AllowObjCConversionOnExplicit=*/false,
                                     /*AllowExplicit*/ true);
  }
}

/// Attempt to convert the given expression to a type which is accepted
/// by the given converter.
///
/// This routine will attempt to convert an expression of class type to a
/// type accepted by the specified converter. In C++11 and before, the class
/// must have a single non-explicit conversion function converting to a matching
/// type. In C++1y, there can be multiple such conversion functions, but only
/// one target type.
///
/// \param Loc The source location of the construct that requires the
/// conversion.
///
/// \param From The expression we're converting from.
///
/// \param Converter Used to control and diagnose the conversion process.
///
/// \returns The expression, converted to an integral or enumeration type if
/// successful.
ExprResult Sema::PerformContextualImplicitConversion(
    SourceLocation Loc, Expr *From, ContextualImplicitConverter &Converter) {
  // We can't perform any more checking for type-dependent expressions.
  if (From->isTypeDependent())
    return From;

  // Process placeholders immediately.
  if (From->hasPlaceholderType()) {
    ExprResult result = CheckPlaceholderExpr(From);
    if (result.isInvalid())
      return result;
    From = result.get();
  }

  // If the expression already has a matching type, we're golden.
  QualType T = From->getType();
  if (Converter.match(T))
    return DefaultLvalueConversion(From);

  // FIXME: Check for missing '()' if T is a function type?

  // We can only perform contextual implicit conversions on objects of class
  // type.
  const RecordType *RecordTy = T->getAs<RecordType>();
  if (!RecordTy || !getLangOpts().CPlusPlus) {
    if (!Converter.Suppress)
      Converter.diagnoseNoMatch(*this, Loc, T) << From->getSourceRange();
    return From;
  }

  // We must have a complete class type.
  struct TypeDiagnoserPartialDiag : TypeDiagnoser {
    ContextualImplicitConverter &Converter;
    Expr *From;

    TypeDiagnoserPartialDiag(ContextualImplicitConverter &Converter, Expr *From)
        : Converter(Converter), From(From) {}

    void diagnose(Sema &S, SourceLocation Loc, QualType T) override {
      Converter.diagnoseIncomplete(S, Loc, T) << From->getSourceRange();
    }
  } IncompleteDiagnoser(Converter, From);

  if (Converter.Suppress ? !isCompleteType(Loc, T)
                         : RequireCompleteType(Loc, T, IncompleteDiagnoser))
    return From;

  // Look for a conversion to an integral or enumeration type.
  UnresolvedSet<4>
      ViableConversions; // These are *potentially* viable in C++1y.
  UnresolvedSet<4> ExplicitConversions;
  const auto &Conversions =
      cast<CXXRecordDecl>(RecordTy->getDecl())->getVisibleConversionFunctions();

  bool HadMultipleCandidates =
      (std::distance(Conversions.begin(), Conversions.end()) > 1);

  // To check that there is only one target type, in C++1y:
  QualType ToType;
  bool HasUniqueTargetType = true;

  // Collect explicit or viable (potentially in C++1y) conversions.
  for (auto I = Conversions.begin(), E = Conversions.end(); I != E; ++I) {
    NamedDecl *D = (*I)->getUnderlyingDecl();
    CXXConversionDecl *Conversion;
    FunctionTemplateDecl *ConvTemplate = dyn_cast<FunctionTemplateDecl>(D);
    if (ConvTemplate) {
      if (getLangOpts().CPlusPlus14)
        Conversion = cast<CXXConversionDecl>(ConvTemplate->getTemplatedDecl());
      else
        continue; // C++11 does not consider conversion operator templates(?).
    } else
      Conversion = cast<CXXConversionDecl>(D);

    assert((!ConvTemplate || getLangOpts().CPlusPlus14) &&
           "Conversion operator templates are considered potentially "
           "viable in C++1y");

    QualType CurToType = Conversion->getConversionType().getNonReferenceType();
    if (Converter.match(CurToType) || ConvTemplate) {

      if (Conversion->isExplicit()) {
        // FIXME: For C++1y, do we need this restriction?
        // cf. diagnoseNoViableConversion()
        if (!ConvTemplate)
          ExplicitConversions.addDecl(I.getDecl(), I.getAccess());
      } else {
        if (!ConvTemplate && getLangOpts().CPlusPlus14) {
          if (ToType.isNull())
            ToType = CurToType.getUnqualifiedType();
          else if (HasUniqueTargetType &&
                   (CurToType.getUnqualifiedType() != ToType))
            HasUniqueTargetType = false;
        }
        ViableConversions.addDecl(I.getDecl(), I.getAccess());
      }
    }
  }

  if (getLangOpts().CPlusPlus14) {
    // C++1y [conv]p6:
    // ... An expression e of class type E appearing in such a context
    // is said to be contextually implicitly converted to a specified
    // type T and is well-formed if and only if e can be implicitly
    // converted to a type T that is determined as follows: E is searched
    // for conversion functions whose return type is cv T or reference to
    // cv T such that T is allowed by the context. There shall be
    // exactly one such T.

    // If no unique T is found:
    if (ToType.isNull()) {
      if (diagnoseNoViableConversion(*this, Loc, From, Converter, T,
                                     HadMultipleCandidates,
                                     ExplicitConversions))
        return ExprError();
      return finishContextualImplicitConversion(*this, Loc, From, Converter);
    }

    // If more than one unique Ts are found:
    if (!HasUniqueTargetType)
      return diagnoseAmbiguousConversion(*this, Loc, From, Converter, T,
                                         ViableConversions);

    // If one unique T is found:
    // First, build a candidate set from the previously recorded
    // potentially viable conversions.
    OverloadCandidateSet CandidateSet(Loc, OverloadCandidateSet::CSK_Normal);
    collectViableConversionCandidates(*this, From, ToType, ViableConversions,
                                      CandidateSet);

    // Then, perform overload resolution over the candidate set.
    OverloadCandidateSet::iterator Best;
    switch (CandidateSet.BestViableFunction(*this, Loc, Best)) {
    case OR_Success: {
      // Apply this conversion.
      DeclAccessPair Found =
          DeclAccessPair::make(Best->Function, Best->FoundDecl.getAccess());
      if (recordConversion(*this, Loc, From, Converter, T,
                           HadMultipleCandidates, Found))
        return ExprError();
      break;
    }
    case OR_Ambiguous:
      return diagnoseAmbiguousConversion(*this, Loc, From, Converter, T,
                                         ViableConversions);
    case OR_No_Viable_Function:
      if (diagnoseNoViableConversion(*this, Loc, From, Converter, T,
                                     HadMultipleCandidates,
                                     ExplicitConversions))
        return ExprError();
      [[fallthrough]];
    case OR_Deleted:
      // We'll complain below about a non-integral condition type.
      break;
    }
  } else {
    switch (ViableConversions.size()) {
    case 0: {
      if (diagnoseNoViableConversion(*this, Loc, From, Converter, T,
                                     HadMultipleCandidates,
                                     ExplicitConversions))
        return ExprError();

      // We'll complain below about a non-integral condition type.
      break;
    }
    case 1: {
      // Apply this conversion.
      DeclAccessPair Found = ViableConversions[0];
      if (recordConversion(*this, Loc, From, Converter, T,
                           HadMultipleCandidates, Found))
        return ExprError();
      break;
    }
    default:
      return diagnoseAmbiguousConversion(*this, Loc, From, Converter, T,
                                         ViableConversions);
    }
  }

  return finishContextualImplicitConversion(*this, Loc, From, Converter);
}

/// IsAcceptableNonMemberOperatorCandidate - Determine whether Fn is
/// an acceptable non-member overloaded operator for a call whose
/// arguments have types T1 (and, if non-empty, T2). This routine
/// implements the check in C++ [over.match.oper]p3b2 concerning
/// enumeration types.
static bool IsAcceptableNonMemberOperatorCandidate(ASTContext &Context,
                                                   FunctionDecl *Fn,
                                                   ArrayRef<Expr *> Args) {
  QualType T1 = Args[0]->getType();
  QualType T2 = Args.size() > 1 ? Args[1]->getType() : QualType();

  if (T1->isDependentType() || (!T2.isNull() && T2->isDependentType()))
    return true;

  if (T1->isRecordType() || (!T2.isNull() && T2->isRecordType()))
    return true;

  const auto *Proto = Fn->getType()->castAs<FunctionProtoType>();
  if (Proto->getNumParams() < 1)
    return false;

  if (T1->isEnumeralType()) {
    QualType ArgType = Proto->getParamType(0).getNonReferenceType();
    if (Context.hasSameUnqualifiedType(T1, ArgType))
      return true;
  }

  if (Proto->getNumParams() < 2)
    return false;

  if (!T2.isNull() && T2->isEnumeralType()) {
    QualType ArgType = Proto->getParamType(1).getNonReferenceType();
    if (Context.hasSameUnqualifiedType(T2, ArgType))
      return true;
  }

  return false;
}

/// AddOverloadCandidate - Adds the given function to the set of
/// candidate functions, using the given function call arguments.  If
/// @p SuppressUserConversions, then don't allow user-defined
/// conversions via constructors or conversion operators.
///
/// \param PartialOverloading true if we are performing "partial" overloading
/// based on an incomplete set of function arguments. This feature is used by
/// code completion.
void Sema::AddOverloadCandidate(
    FunctionDecl *Function, DeclAccessPair FoundDecl, ArrayRef<Expr *> Args,
    OverloadCandidateSet &CandidateSet, bool SuppressUserConversions,
    bool PartialOverloading, bool AllowExplicit, bool AllowExplicitConversions,
    ADLCallKind IsADLCandidate, ConversionSequenceList EarlyConversions,
    OverloadCandidateParamOrder PO) {
  const FunctionProtoType *Proto
    = dyn_cast<FunctionProtoType>(Function->getType()->getAs<FunctionType>());
  assert(Proto && "Functions without a prototype cannot be overloaded");
  assert(!Function->getDescribedFunctionTemplate() &&
         "Use AddTemplateOverloadCandidate for function templates");

  if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Function)) {
    if (!isa<CXXConstructorDecl>(Method)) {
      // If we get here, it's because we're calling a member function
      // that is named without a member access expression (e.g.,
      // "this->f") that was either written explicitly or created
      // implicitly. This can happen with a qualified call to a member
      // function, e.g., X::f(). We use an empty type for the implied
      // object argument (C++ [over.call.func]p3), and the acting context
      // is irrelevant.
      AddMethodCandidate(Method, FoundDecl, Method->getParent(), QualType(),
                         Expr::Classification::makeSimpleLValue(), Args,
                         CandidateSet, SuppressUserConversions,
                         PartialOverloading, EarlyConversions, PO);
      return;
    }
    // We treat a constructor like a non-member function, since its object
    // argument doesn't participate in overload resolution.
  }

  if (!CandidateSet.isNewCandidate(Function, PO))
    return;

  // C++11 [class.copy]p11: [DR1402]
  //   A defaulted move constructor that is defined as deleted is ignored by
  //   overload resolution.
  CXXConstructorDecl *Constructor = dyn_cast<CXXConstructorDecl>(Function);
  if (Constructor && Constructor->isDefaulted() && Constructor->isDeleted() &&
      Constructor->isMoveConstructor())
    return;

  // Overload resolution is always an unevaluated context.
  EnterExpressionEvaluationContext Unevaluated(
      *this, Sema::ExpressionEvaluationContext::Unevaluated);

  // C++ [over.match.oper]p3:
  //   if no operand has a class type, only those non-member functions in the
  //   lookup set that have a first parameter of type T1 or "reference to
  //   (possibly cv-qualified) T1", when T1 is an enumeration type, or (if there
  //   is a right operand) a second parameter of type T2 or "reference to
  //   (possibly cv-qualified) T2", when T2 is an enumeration type, are
  //   candidate functions.
  if (CandidateSet.getKind() == OverloadCandidateSet::CSK_Operator &&
      !IsAcceptableNonMemberOperatorCandidate(Context, Function, Args))
    return;

  // Add this candidate
  OverloadCandidate &Candidate =
      CandidateSet.addCandidate(Args.size(), EarlyConversions);
  Candidate.FoundDecl = FoundDecl;
  Candidate.Function = Function;
  Candidate.Viable = true;
  Candidate.RewriteKind =
      CandidateSet.getRewriteInfo().getRewriteKind(Function, PO);
  Candidate.IsSurrogate = false;
  Candidate.IsADLCandidate = IsADLCandidate;
  Candidate.IgnoreObjectArgument = false;
  Candidate.ExplicitCallArguments = Args.size();

  // Explicit functions are not actually candidates at all if we're not
  // allowing them in this context, but keep them around so we can point
  // to them in diagnostics.
  if (!AllowExplicit && ExplicitSpecifier::getFromDecl(Function).isExplicit()) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_explicit;
    return;
  }

  // Functions with internal linkage are only viable in the same module unit.
  if (auto *MF = Function->getOwningModule()) {
    if (getLangOpts().CPlusPlusModules && !MF->isModuleMapModule() &&
        !isModuleUnitOfCurrentTU(MF)) {
      /// FIXME: Currently, the semantics of linkage in clang is slightly
      /// different from the semantics in C++ spec. In C++ spec, only names
      /// have linkage. So that all entities of the same should share one
      /// linkage. But in clang, different entities of the same could have
      /// different linkage.
      NamedDecl *ND = Function;
      if (auto *SpecInfo = Function->getTemplateSpecializationInfo())
        ND = SpecInfo->getTemplate();
      
      if (ND->getFormalLinkage() == Linkage::InternalLinkage) {
        Candidate.Viable = false;
        Candidate.FailureKind = ovl_fail_module_mismatched;
        return;
      }
    }
  }

  if (Function->isMultiVersion() &&
      ((Function->hasAttr<TargetAttr>() &&
        !Function->getAttr<TargetAttr>()->isDefaultVersion()) ||
       (Function->hasAttr<TargetVersionAttr>() &&
        !Function->getAttr<TargetVersionAttr>()->isDefaultVersion()))) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_non_default_multiversion_function;
    return;
  }

  if (Constructor) {
    // C++ [class.copy]p3:
    //   A member function template is never instantiated to perform the copy
    //   of a class object to an object of its class type.
    QualType ClassType = Context.getTypeDeclType(Constructor->getParent());
    if (Args.size() == 1 && Constructor->isSpecializationCopyingObject() &&
        (Context.hasSameUnqualifiedType(ClassType, Args[0]->getType()) ||
         IsDerivedFrom(Args[0]->getBeginLoc(), Args[0]->getType(),
                       ClassType))) {
      Candidate.Viable = false;
      Candidate.FailureKind = ovl_fail_illegal_constructor;
      return;
    }

    // C++ [over.match.funcs]p8: (proposed DR resolution)
    //   A constructor inherited from class type C that has a first parameter
    //   of type "reference to P" (including such a constructor instantiated
    //   from a template) is excluded from the set of candidate functions when
    //   constructing an object of type cv D if the argument list has exactly
    //   one argument and D is reference-related to P and P is reference-related
    //   to C.
    auto *Shadow = dyn_cast<ConstructorUsingShadowDecl>(FoundDecl.getDecl());
    if (Shadow && Args.size() == 1 && Constructor->getNumParams() >= 1 &&
        Constructor->getParamDecl(0)->getType()->isReferenceType()) {
      QualType P = Constructor->getParamDecl(0)->getType()->getPointeeType();
      QualType C = Context.getRecordType(Constructor->getParent());
      QualType D = Context.getRecordType(Shadow->getParent());
      SourceLocation Loc = Args.front()->getExprLoc();
      if ((Context.hasSameUnqualifiedType(P, C) || IsDerivedFrom(Loc, P, C)) &&
          (Context.hasSameUnqualifiedType(D, P) || IsDerivedFrom(Loc, D, P))) {
        Candidate.Viable = false;
        Candidate.FailureKind = ovl_fail_inhctor_slice;
        return;
      }
    }

    // Check that the constructor is capable of constructing an object in the
    // destination address space.
    if (!Qualifiers::isAddressSpaceSupersetOf(
            Constructor->getMethodQualifiers().getAddressSpace(),
            CandidateSet.getDestAS())) {
      Candidate.Viable = false;
      Candidate.FailureKind = ovl_fail_object_addrspace_mismatch;
    }
  }

  unsigned NumParams = Proto->getNumParams();

  // (C++ 13.3.2p2): A candidate function having fewer than m
  // parameters is viable only if it has an ellipsis in its parameter
  // list (8.3.5).
  if (TooManyArguments(NumParams, Args.size(), PartialOverloading) &&
      !Proto->isVariadic() &&
      shouldEnforceArgLimit(PartialOverloading, Function)) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_too_many_arguments;
    return;
  }

  // (C++ 13.3.2p2): A candidate function having more than m parameters
  // is viable only if the (m+1)st parameter has a default argument
  // (8.3.6). For the purposes of overload resolution, the
  // parameter list is truncated on the right, so that there are
  // exactly m parameters.
  unsigned MinRequiredArgs = Function->getMinRequiredArguments();
  if (Args.size() < MinRequiredArgs && !PartialOverloading) {
    // Not enough arguments.
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_too_few_arguments;
    return;
  }

  // (CUDA B.1): Check for invalid calls between targets.
  if (getLangOpts().CUDA)
    if (const FunctionDecl *Caller = getCurFunctionDecl(/*AllowLambda=*/true))
      // Skip the check for callers that are implicit members, because in this
      // case we may not yet know what the member's target is; the target is
      // inferred for the member automatically, based on the bases and fields of
      // the class.
      if (!Caller->isImplicit() && !IsAllowedCUDACall(Caller, Function)) {
        Candidate.Viable = false;
        Candidate.FailureKind = ovl_fail_bad_target;
        return;
      }

  if (Function->getTrailingRequiresClause()) {
    ConstraintSatisfaction Satisfaction;
    if (CheckFunctionConstraints(Function, Satisfaction, /*Loc*/ {},
                                 /*ForOverloadResolution*/ true) ||
        !Satisfaction.IsSatisfied) {
      Candidate.Viable = false;
      Candidate.FailureKind = ovl_fail_constraints_not_satisfied;
      return;
    }
  }

  // Determine the implicit conversion sequences for each of the
  // arguments.
  for (unsigned ArgIdx = 0; ArgIdx < Args.size(); ++ArgIdx) {
    unsigned ConvIdx =
        PO == OverloadCandidateParamOrder::Reversed ? 1 - ArgIdx : ArgIdx;
    if (Candidate.Conversions[ConvIdx].isInitialized()) {
      // We already formed a conversion sequence for this parameter during
      // template argument deduction.
    } else if (ArgIdx < NumParams) {
      // (C++ 13.3.2p3): for F to be a viable function, there shall
      // exist for each argument an implicit conversion sequence
      // (13.3.3.1) that converts that argument to the corresponding
      // parameter of F.
      QualType ParamType = Proto->getParamType(ArgIdx);
      Candidate.Conversions[ConvIdx] = TryCopyInitialization(
          *this, Args[ArgIdx], ParamType, SuppressUserConversions,
          /*InOverloadResolution=*/true,
          /*AllowObjCWritebackConversion=*/
          getLangOpts().ObjCAutoRefCount, AllowExplicitConversions);
      if (Candidate.Conversions[ConvIdx].isBad()) {
        Candidate.Viable = false;
        Candidate.FailureKind = ovl_fail_bad_conversion;
        return;
      }
    } else {
      // (C++ 13.3.2p2): For the purposes of overload resolution, any
      // argument for which there is no corresponding parameter is
      // considered to ""match the ellipsis" (C+ 13.3.3.1.3).
      Candidate.Conversions[ConvIdx].setEllipsis();
    }
  }

  if (EnableIfAttr *FailedAttr =
          CheckEnableIf(Function, CandidateSet.getLocation(), Args)) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_enable_if;
    Candidate.DeductionFailure.Data = FailedAttr;
    return;
  }
}

ObjCMethodDecl *
Sema::SelectBestMethod(Selector Sel, MultiExprArg Args, bool IsInstance,
                       SmallVectorImpl<ObjCMethodDecl *> &Methods) {
  if (Methods.size() <= 1)
    return nullptr;

  for (unsigned b = 0, e = Methods.size(); b < e; b++) {
    bool Match = true;
    ObjCMethodDecl *Method = Methods[b];
    unsigned NumNamedArgs = Sel.getNumArgs();
    // Method might have more arguments than selector indicates. This is due
    // to addition of c-style arguments in method.
    if (Method->param_size() > NumNamedArgs)
      NumNamedArgs = Method->param_size();
    if (Args.size() < NumNamedArgs)
      continue;

    for (unsigned i = 0; i < NumNamedArgs; i++) {
      // We can't do any type-checking on a type-dependent argument.
      if (Args[i]->isTypeDependent()) {
        Match = false;
        break;
      }

      ParmVarDecl *param = Method->parameters()[i];
      Expr *argExpr = Args[i];
      assert(argExpr && "SelectBestMethod(): missing expression");

      // Strip the unbridged-cast placeholder expression off unless it's
      // a consumed argument.
      if (argExpr->hasPlaceholderType(BuiltinType::ARCUnbridgedCast) &&
          !param->hasAttr<CFConsumedAttr>())
        argExpr = stripARCUnbridgedCast(argExpr);

      // If the parameter is __unknown_anytype, move on to the next method.
      if (param->getType() == Context.UnknownAnyTy) {
        Match = false;
        break;
      }

      ImplicitConversionSequence ConversionState
        = TryCopyInitialization(*this, argExpr, param->getType(),
                                /*SuppressUserConversions*/false,
                                /*InOverloadResolution=*/true,
                                /*AllowObjCWritebackConversion=*/
                                getLangOpts().ObjCAutoRefCount,
                                /*AllowExplicit*/false);
      // This function looks for a reasonably-exact match, so we consider
      // incompatible pointer conversions to be a failure here.
      if (ConversionState.isBad() ||
          (ConversionState.isStandard() &&
           ConversionState.Standard.Second ==
               ICK_Incompatible_Pointer_Conversion)) {
        Match = false;
        break;
      }
    }
    // Promote additional arguments to variadic methods.
    if (Match && Method->isVariadic()) {
      for (unsigned i = NumNamedArgs, e = Args.size(); i < e; ++i) {
        if (Args[i]->isTypeDependent()) {
          Match = false;
          break;
        }
        ExprResult Arg = DefaultVariadicArgumentPromotion(Args[i], VariadicMethod,
                                                          nullptr);
        if (Arg.isInvalid()) {
          Match = false;
          break;
        }
      }
    } else {
      // Check for extra arguments to non-variadic methods.
      if (Args.size() != NumNamedArgs)
        Match = false;
      else if (Match && NumNamedArgs == 0 && Methods.size() > 1) {
        // Special case when selectors have no argument. In this case, select
        // one with the most general result type of 'id'.
        for (unsigned b = 0, e = Methods.size(); b < e; b++) {
          QualType ReturnT = Methods[b]->getReturnType();
          if (ReturnT->isObjCIdType())
            return Methods[b];
        }
      }
    }

    if (Match)
      return Method;
  }
  return nullptr;
}

static bool convertArgsForAvailabilityChecks(
    Sema &S, FunctionDecl *Function, Expr *ThisArg, SourceLocation CallLoc,
    ArrayRef<Expr *> Args, Sema::SFINAETrap &Trap, bool MissingImplicitThis,
    Expr *&ConvertedThis, SmallVectorImpl<Expr *> &ConvertedArgs) {
  if (ThisArg) {
    CXXMethodDecl *Method = cast<CXXMethodDecl>(Function);
    assert(!isa<CXXConstructorDecl>(Method) &&
           "Shouldn't have `this` for ctors!");
    assert(!Method->isStatic() && "Shouldn't have `this` for static methods!");
    ExprResult R = S.PerformObjectArgumentInitialization(
        ThisArg, /*Qualifier=*/nullptr, Method, Method);
    if (R.isInvalid())
      return false;
    ConvertedThis = R.get();
  } else {
    if (auto *MD = dyn_cast<CXXMethodDecl>(Function)) {
      (void)MD;
      assert((MissingImplicitThis || MD->isStatic() ||
              isa<CXXConstructorDecl>(MD)) &&
             "Expected `this` for non-ctor instance methods");
    }
    ConvertedThis = nullptr;
  }

  // Ignore any variadic arguments. Converting them is pointless, since the
  // user can't refer to them in the function condition.
  unsigned ArgSizeNoVarargs = std::min(Function->param_size(), Args.size());

  // Convert the arguments.
  for (unsigned I = 0; I != ArgSizeNoVarargs; ++I) {
    ExprResult R;
    R = S.PerformCopyInitialization(InitializedEntity::InitializeParameter(
                                        S.Context, Function->getParamDecl(I)),
                                    SourceLocation(), Args[I]);

    if (R.isInvalid())
      return false;

    ConvertedArgs.push_back(R.get());
  }

  if (Trap.hasErrorOccurred())
    return false;

  // Push default arguments if needed.
  if (!Function->isVariadic() && Args.size() < Function->getNumParams()) {
    for (unsigned i = Args.size(), e = Function->getNumParams(); i != e; ++i) {
      ParmVarDecl *P = Function->getParamDecl(i);
      if (!P->hasDefaultArg())
        return false;
      ExprResult R = S.BuildCXXDefaultArgExpr(CallLoc, Function, P);
      if (R.isInvalid())
        return false;
      ConvertedArgs.push_back(R.get());
    }

    if (Trap.hasErrorOccurred())
      return false;
  }
  return true;
}

EnableIfAttr *Sema::CheckEnableIf(FunctionDecl *Function,
                                  SourceLocation CallLoc,
                                  ArrayRef<Expr *> Args,
                                  bool MissingImplicitThis) {
  auto EnableIfAttrs = Function->specific_attrs<EnableIfAttr>();
  if (EnableIfAttrs.begin() == EnableIfAttrs.end())
    return nullptr;

  SFINAETrap Trap(*this);
  SmallVector<Expr *, 16> ConvertedArgs;
  // FIXME: We should look into making enable_if late-parsed.
  Expr *DiscardedThis;
  if (!convertArgsForAvailabilityChecks(
          *this, Function, /*ThisArg=*/nullptr, CallLoc, Args, Trap,
          /*MissingImplicitThis=*/true, DiscardedThis, ConvertedArgs))
    return *EnableIfAttrs.begin();

  for (auto *EIA : EnableIfAttrs) {
    APValue Result;
    // FIXME: This doesn't consider value-dependent cases, because doing so is
    // very difficult. Ideally, we should handle them more gracefully.
    if (EIA->getCond()->isValueDependent() ||
        !EIA->getCond()->EvaluateWithSubstitution(
            Result, Context, Function, llvm::ArrayRef(ConvertedArgs)))
      return EIA;

    if (!Result.isInt() || !Result.getInt().getBoolValue())
      return EIA;
  }
  return nullptr;
}

template <typename CheckFn>
static bool diagnoseDiagnoseIfAttrsWith(Sema &S, const NamedDecl *ND,
                                        bool ArgDependent, SourceLocation Loc,
                                        CheckFn &&IsSuccessful) {
  SmallVector<const DiagnoseIfAttr *, 8> Attrs;
  for (const auto *DIA : ND->specific_attrs<DiagnoseIfAttr>()) {
    if (ArgDependent == DIA->getArgDependent())
      Attrs.push_back(DIA);
  }

  // Common case: No diagnose_if attributes, so we can quit early.
  if (Attrs.empty())
    return false;

  auto WarningBegin = std::stable_partition(
      Attrs.begin(), Attrs.end(),
      [](const DiagnoseIfAttr *DIA) { return DIA->isError(); });

  // Note that diagnose_if attributes are late-parsed, so they appear in the
  // correct order (unlike enable_if attributes).
  auto ErrAttr = llvm::find_if(llvm::make_range(Attrs.begin(), WarningBegin),
                               IsSuccessful);
  if (ErrAttr != WarningBegin) {
    const DiagnoseIfAttr *DIA = *ErrAttr;
    S.Diag(Loc, diag::err_diagnose_if_succeeded) << DIA->getMessage();
    S.Diag(DIA->getLocation(), diag::note_from_diagnose_if)
        << DIA->getParent() << DIA->getCond()->getSourceRange();
    return true;
  }

  for (const auto *DIA : llvm::make_range(WarningBegin, Attrs.end()))
    if (IsSuccessful(DIA)) {
      S.Diag(Loc, diag::warn_diagnose_if_succeeded) << DIA->getMessage();
      S.Diag(DIA->getLocation(), diag::note_from_diagnose_if)
          << DIA->getParent() << DIA->getCond()->getSourceRange();
    }

  return false;
}

bool Sema::diagnoseArgDependentDiagnoseIfAttrs(const FunctionDecl *Function,
                                               const Expr *ThisArg,
                                               ArrayRef<const Expr *> Args,
                                               SourceLocation Loc) {
  return diagnoseDiagnoseIfAttrsWith(
      *this, Function, /*ArgDependent=*/true, Loc,
      [&](const DiagnoseIfAttr *DIA) {
        APValue Result;
        // It's sane to use the same Args for any redecl of this function, since
        // EvaluateWithSubstitution only cares about the position of each
        // argument in the arg list, not the ParmVarDecl* it maps to.
        if (!DIA->getCond()->EvaluateWithSubstitution(
                Result, Context, cast<FunctionDecl>(DIA->getParent()), Args, ThisArg))
          return false;
        return Result.isInt() && Result.getInt().getBoolValue();
      });
}

bool Sema::diagnoseArgIndependentDiagnoseIfAttrs(const NamedDecl *ND,
                                                 SourceLocation Loc) {
  return diagnoseDiagnoseIfAttrsWith(
      *this, ND, /*ArgDependent=*/false, Loc,
      [&](const DiagnoseIfAttr *DIA) {
        bool Result;
        return DIA->getCond()->EvaluateAsBooleanCondition(Result, Context) &&
               Result;
      });
}

/// Add all of the function declarations in the given function set to
/// the overload candidate set.
void Sema::AddFunctionCandidates(const UnresolvedSetImpl &Fns,
                                 ArrayRef<Expr *> Args,
                                 OverloadCandidateSet &CandidateSet,
                                 TemplateArgumentListInfo *ExplicitTemplateArgs,
                                 bool SuppressUserConversions,
                                 bool PartialOverloading,
                                 bool FirstArgumentIsBase) {
  for (UnresolvedSetIterator F = Fns.begin(), E = Fns.end(); F != E; ++F) {
    NamedDecl *D = F.getDecl()->getUnderlyingDecl();
    ArrayRef<Expr *> FunctionArgs = Args;

    FunctionTemplateDecl *FunTmpl = dyn_cast<FunctionTemplateDecl>(D);
    FunctionDecl *FD =
        FunTmpl ? FunTmpl->getTemplatedDecl() : cast<FunctionDecl>(D);

    if (isa<CXXMethodDecl>(FD) && !cast<CXXMethodDecl>(FD)->isStatic()) {
      QualType ObjectType;
      Expr::Classification ObjectClassification;
      if (Args.size() > 0) {
        if (Expr *E = Args[0]) {
          // Use the explicit base to restrict the lookup:
          ObjectType = E->getType();
          // Pointers in the object arguments are implicitly dereferenced, so we
          // always classify them as l-values.
          if (!ObjectType.isNull() && ObjectType->isPointerType())
            ObjectClassification = Expr::Classification::makeSimpleLValue();
          else
            ObjectClassification = E->Classify(Context);
        } // .. else there is an implicit base.
        FunctionArgs = Args.slice(1);
      }
      if (FunTmpl) {
        AddMethodTemplateCandidate(
            FunTmpl, F.getPair(),
            cast<CXXRecordDecl>(FunTmpl->getDeclContext()),
            ExplicitTemplateArgs, ObjectType, ObjectClassification,
            FunctionArgs, CandidateSet, SuppressUserConversions,
            PartialOverloading);
      } else {
        AddMethodCandidate(cast<CXXMethodDecl>(FD), F.getPair(),
                           cast<CXXMethodDecl>(FD)->getParent(), ObjectType,
                           ObjectClassification, FunctionArgs, CandidateSet,
                           SuppressUserConversions, PartialOverloading);
      }
    } else {
      // This branch handles both standalone functions and static methods.

      // Slice the first argument (which is the base) when we access
      // static method as non-static.
      if (Args.size() > 0 &&
          (!Args[0] || (FirstArgumentIsBase && isa<CXXMethodDecl>(FD) &&
                        !isa<CXXConstructorDecl>(FD)))) {
        assert(cast<CXXMethodDecl>(FD)->isStatic());
        FunctionArgs = Args.slice(1);
      }
      if (FunTmpl) {
        AddTemplateOverloadCandidate(FunTmpl, F.getPair(),
                                     ExplicitTemplateArgs, FunctionArgs,
                                     CandidateSet, SuppressUserConversions,
                                     PartialOverloading);
      } else {
        AddOverloadCandidate(FD, F.getPair(), FunctionArgs, CandidateSet,
                             SuppressUserConversions, PartialOverloading);
      }
    }
  }
}

/// AddMethodCandidate - Adds a named decl (which is some kind of
/// method) as a method candidate to the given overload set.
void Sema::AddMethodCandidate(DeclAccessPair FoundDecl, QualType ObjectType,
                              Expr::Classification ObjectClassification,
                              ArrayRef<Expr *> Args,
                              OverloadCandidateSet &CandidateSet,
                              bool SuppressUserConversions,
                              OverloadCandidateParamOrder PO) {
  NamedDecl *Decl = FoundDecl.getDecl();
  CXXRecordDecl *ActingContext = cast<CXXRecordDecl>(Decl->getDeclContext());

  if (isa<UsingShadowDecl>(Decl))
    Decl = cast<UsingShadowDecl>(Decl)->getTargetDecl();

  if (FunctionTemplateDecl *TD = dyn_cast<FunctionTemplateDecl>(Decl)) {
    assert(isa<CXXMethodDecl>(TD->getTemplatedDecl()) &&
           "Expected a member function template");
    AddMethodTemplateCandidate(TD, FoundDecl, ActingContext,
                               /*ExplicitArgs*/ nullptr, ObjectType,
                               ObjectClassification, Args, CandidateSet,
                               SuppressUserConversions, false, PO);
  } else {
    AddMethodCandidate(cast<CXXMethodDecl>(Decl), FoundDecl, ActingContext,
                       ObjectType, ObjectClassification, Args, CandidateSet,
                       SuppressUserConversions, false, std::nullopt, PO);
  }
}

/// AddMethodCandidate - Adds the given C++ member function to the set
/// of candidate functions, using the given function call arguments
/// and the object argument (@c Object). For example, in a call
/// @c o.f(a1,a2), @c Object will contain @c o and @c Args will contain
/// both @c a1 and @c a2. If @p SuppressUserConversions, then don't
/// allow user-defined conversions via constructors or conversion
/// operators.
void
Sema::AddMethodCandidate(CXXMethodDecl *Method, DeclAccessPair FoundDecl,
                         CXXRecordDecl *ActingContext, QualType ObjectType,
                         Expr::Classification ObjectClassification,
                         ArrayRef<Expr *> Args,
                         OverloadCandidateSet &CandidateSet,
                         bool SuppressUserConversions,
                         bool PartialOverloading,
                         ConversionSequenceList EarlyConversions,
                         OverloadCandidateParamOrder PO) {
  const FunctionProtoType *Proto
    = dyn_cast<FunctionProtoType>(Method->getType()->getAs<FunctionType>());
  assert(Proto && "Methods without a prototype cannot be overloaded");
  assert(!isa<CXXConstructorDecl>(Method) &&
         "Use AddOverloadCandidate for constructors");

  if (!CandidateSet.isNewCandidate(Method, PO))
    return;

  // C++11 [class.copy]p23: [DR1402]
  //   A defaulted move assignment operator that is defined as deleted is
  //   ignored by overload resolution.
  if (Method->isDefaulted() && Method->isDeleted() &&
      Method->isMoveAssignmentOperator())
    return;

  // Overload resolution is always an unevaluated context.
  EnterExpressionEvaluationContext Unevaluated(
      *this, Sema::ExpressionEvaluationContext::Unevaluated);

  // Add this candidate
  OverloadCandidate &Candidate =
      CandidateSet.addCandidate(Args.size() + 1, EarlyConversions);
  Candidate.FoundDecl = FoundDecl;
  Candidate.Function = Method;
  Candidate.RewriteKind =
      CandidateSet.getRewriteInfo().getRewriteKind(Method, PO);
  Candidate.IsSurrogate = false;
  Candidate.IgnoreObjectArgument = false;
  Candidate.ExplicitCallArguments = Args.size();

  unsigned NumParams = Proto->getNumParams();

  // (C++ 13.3.2p2): A candidate function having fewer than m
  // parameters is viable only if it has an ellipsis in its parameter
  // list (8.3.5).
  if (TooManyArguments(NumParams, Args.size(), PartialOverloading) &&
      !Proto->isVariadic() &&
      shouldEnforceArgLimit(PartialOverloading, Method)) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_too_many_arguments;
    return;
  }

  // (C++ 13.3.2p2): A candidate function having more than m parameters
  // is viable only if the (m+1)st parameter has a default argument
  // (8.3.6). For the purposes of overload resolution, the
  // parameter list is truncated on the right, so that there are
  // exactly m parameters.
  unsigned MinRequiredArgs = Method->getMinRequiredArguments();
  if (Args.size() < MinRequiredArgs && !PartialOverloading) {
    // Not enough arguments.
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_too_few_arguments;
    return;
  }

  Candidate.Viable = true;

  unsigned FirstConvIdx = PO == OverloadCandidateParamOrder::Reversed ? 1 : 0;
  if (ObjectType.isNull())
    Candidate.IgnoreObjectArgument = true;
  else if (Method->isStatic()) {
    // [over.best.ics.general]p8
    // When the parameter is the implicit object parameter of a static member
    // function, the implicit conversion sequence is a standard conversion
    // sequence that is neither better nor worse than any other standard
    // conversion sequence.
    //
    // This is a rule that was introduced in C++23 to support static lambdas. We
    // apply it retroactively because we want to support static lambdas as an
    // extension and it doesn't hurt previous code.
    Candidate.Conversions[FirstConvIdx].setStaticObjectArgument();
  } else {
    // Determine the implicit conversion sequence for the object
    // parameter.
    Candidate.Conversions[FirstConvIdx] = TryObjectArgumentInitialization(
        *this, CandidateSet.getLocation(), ObjectType, ObjectClassification,
        Method, ActingContext);
    if (Candidate.Conversions[FirstConvIdx].isBad()) {
      Candidate.Viable = false;
      Candidate.FailureKind = ovl_fail_bad_conversion;
      return;
    }
  }

  // (CUDA B.1): Check for invalid calls between targets.
  if (getLangOpts().CUDA)
    if (const FunctionDecl *Caller = getCurFunctionDecl(/*AllowLambda=*/true))
      if (!IsAllowedCUDACall(Caller, Method)) {
        Candidate.Viable = false;
        Candidate.FailureKind = ovl_fail_bad_target;
        return;
      }

  if (Method->getTrailingRequiresClause()) {
    ConstraintSatisfaction Satisfaction;
    if (CheckFunctionConstraints(Method, Satisfaction, /*Loc*/ {},
                                 /*ForOverloadResolution*/ true) ||
        !Satisfaction.IsSatisfied) {
      Candidate.Viable = false;
      Candidate.FailureKind = ovl_fail_constraints_not_satisfied;
      return;
    }
  }

  // Determine the implicit conversion sequences for each of the
  // arguments.
  for (unsigned ArgIdx = 0; ArgIdx < Args.size(); ++ArgIdx) {
    unsigned ConvIdx =
        PO == OverloadCandidateParamOrder::Reversed ? 0 : (ArgIdx + 1);
    if (Candidate.Conversions[ConvIdx].isInitialized()) {
      // We already formed a conversion sequence for this parameter during
      // template argument deduction.
    } else if (ArgIdx < NumParams) {
      // (C++ 13.3.2p3): for F to be a viable function, there shall
      // exist for each argument an implicit conversion sequence
      // (13.3.3.1) that converts that argument to the corresponding
      // parameter of F.
      QualType ParamType = Proto->getParamType(ArgIdx);
      Candidate.Conversions[ConvIdx]
        = TryCopyInitialization(*this, Args[ArgIdx], ParamType,
                                SuppressUserConversions,
                                /*InOverloadResolution=*/true,
                                /*AllowObjCWritebackConversion=*/
                                  getLangOpts().ObjCAutoRefCount);
      if (Candidate.Conversions[ConvIdx].isBad()) {
        Candidate.Viable = false;
        Candidate.FailureKind = ovl_fail_bad_conversion;
        return;
      }
    } else {
      // (C++ 13.3.2p2): For the purposes of overload resolution, any
      // argument for which there is no corresponding parameter is
      // considered to "match the ellipsis" (C+ 13.3.3.1.3).
      Candidate.Conversions[ConvIdx].setEllipsis();
    }
  }

  if (EnableIfAttr *FailedAttr =
          CheckEnableIf(Method, CandidateSet.getLocation(), Args, true)) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_enable_if;
    Candidate.DeductionFailure.Data = FailedAttr;
    return;
  }

  if (Method->isMultiVersion() &&
      ((Method->hasAttr<TargetAttr>() &&
        !Method->getAttr<TargetAttr>()->isDefaultVersion()) ||
       (Method->hasAttr<TargetVersionAttr>() &&
        !Method->getAttr<TargetVersionAttr>()->isDefaultVersion()))) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_non_default_multiversion_function;
  }
}

/// Add a C++ member function template as a candidate to the candidate
/// set, using template argument deduction to produce an appropriate member
/// function template specialization.
void Sema::AddMethodTemplateCandidate(
    FunctionTemplateDecl *MethodTmpl, DeclAccessPair FoundDecl,
    CXXRecordDecl *ActingContext,
    TemplateArgumentListInfo *ExplicitTemplateArgs, QualType ObjectType,
    Expr::Classification ObjectClassification, ArrayRef<Expr *> Args,
    OverloadCandidateSet &CandidateSet, bool SuppressUserConversions,
    bool PartialOverloading, OverloadCandidateParamOrder PO) {
  if (!CandidateSet.isNewCandidate(MethodTmpl, PO))
    return;

  // C++ [over.match.funcs]p7:
  //   In each case where a candidate is a function template, candidate
  //   function template specializations are generated using template argument
  //   deduction (14.8.3, 14.8.2). Those candidates are then handled as
  //   candidate functions in the usual way.113) A given name can refer to one
  //   or more function templates and also to a set of overloaded non-template
  //   functions. In such a case, the candidate functions generated from each
  //   function template are combined with the set of non-template candidate
  //   functions.
  TemplateDeductionInfo Info(CandidateSet.getLocation());
  FunctionDecl *Specialization = nullptr;
  ConversionSequenceList Conversions;
  if (TemplateDeductionResult Result = DeduceTemplateArguments(
          MethodTmpl, ExplicitTemplateArgs, Args, Specialization, Info,
          PartialOverloading, [&](ArrayRef<QualType> ParamTypes) {
            return CheckNonDependentConversions(
                MethodTmpl, ParamTypes, Args, CandidateSet, Conversions,
                SuppressUserConversions, ActingContext, ObjectType,
                ObjectClassification, PO);
          })) {
    OverloadCandidate &Candidate =
        CandidateSet.addCandidate(Conversions.size(), Conversions);
    Candidate.FoundDecl = FoundDecl;
    Candidate.Function = MethodTmpl->getTemplatedDecl();
    Candidate.Viable = false;
    Candidate.RewriteKind =
      CandidateSet.getRewriteInfo().getRewriteKind(Candidate.Function, PO);
    Candidate.IsSurrogate = false;
    Candidate.IgnoreObjectArgument =
        cast<CXXMethodDecl>(Candidate.Function)->isStatic() ||
        ObjectType.isNull();
    Candidate.ExplicitCallArguments = Args.size();
    if (Result == TDK_NonDependentConversionFailure)
      Candidate.FailureKind = ovl_fail_bad_conversion;
    else {
      Candidate.FailureKind = ovl_fail_bad_deduction;
      Candidate.DeductionFailure = MakeDeductionFailureInfo(Context, Result,
                                                            Info);
    }
    return;
  }

  // Add the function template specialization produced by template argument
  // deduction as a candidate.
  assert(Specialization && "Missing member function template specialization?");
  assert(isa<CXXMethodDecl>(Specialization) &&
         "Specialization is not a member function?");
  AddMethodCandidate(cast<CXXMethodDecl>(Specialization), FoundDecl,
                     ActingContext, ObjectType, ObjectClassification, Args,
                     CandidateSet, SuppressUserConversions, PartialOverloading,
                     Conversions, PO);
}

/// Determine whether a given function template has a simple explicit specifier
/// or a non-value-dependent explicit-specification that evaluates to true.
static bool isNonDependentlyExplicit(FunctionTemplateDecl *FTD) {
  return ExplicitSpecifier::getFromDecl(FTD->getTemplatedDecl()).isExplicit();
}

/// Add a C++ function template specialization as a candidate
/// in the candidate set, using template argument deduction to produce
/// an appropriate function template specialization.
void Sema::AddTemplateOverloadCandidate(
    FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
    TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
    OverloadCandidateSet &CandidateSet, bool SuppressUserConversions,
    bool PartialOverloading, bool AllowExplicit, ADLCallKind IsADLCandidate,
    OverloadCandidateParamOrder PO) {
  if (!CandidateSet.isNewCandidate(FunctionTemplate, PO))
    return;

  // If the function template has a non-dependent explicit specification,
  // exclude it now if appropriate; we are not permitted to perform deduction
  // and substitution in this case.
  if (!AllowExplicit && isNonDependentlyExplicit(FunctionTemplate)) {
    OverloadCandidate &Candidate = CandidateSet.addCandidate();
    Candidate.FoundDecl = FoundDecl;
    Candidate.Function = FunctionTemplate->getTemplatedDecl();
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_explicit;
    return;
  }

  // C++ [over.match.funcs]p7:
  //   In each case where a candidate is a function template, candidate
  //   function template specializations are generated using template argument
  //   deduction (14.8.3, 14.8.2). Those candidates are then handled as
  //   candidate functions in the usual way.113) A given name can refer to one
  //   or more function templates and also to a set of overloaded non-template
  //   functions. In such a case, the candidate functions generated from each
  //   function template are combined with the set of non-template candidate
  //   functions.
  TemplateDeductionInfo Info(CandidateSet.getLocation());
  FunctionDecl *Specialization = nullptr;
  ConversionSequenceList Conversions;
  if (TemplateDeductionResult Result = DeduceTemplateArguments(
          FunctionTemplate, ExplicitTemplateArgs, Args, Specialization, Info,
          PartialOverloading, [&](ArrayRef<QualType> ParamTypes) {
            return CheckNonDependentConversions(
                FunctionTemplate, ParamTypes, Args, CandidateSet, Conversions,
                SuppressUserConversions, nullptr, QualType(), {}, PO);
          })) {
    OverloadCandidate &Candidate =
        CandidateSet.addCandidate(Conversions.size(), Conversions);
    Candidate.FoundDecl = FoundDecl;
    Candidate.Function = FunctionTemplate->getTemplatedDecl();
    Candidate.Viable = false;
    Candidate.RewriteKind =
      CandidateSet.getRewriteInfo().getRewriteKind(Candidate.Function, PO);
    Candidate.IsSurrogate = false;
    Candidate.IsADLCandidate = IsADLCandidate;
    // Ignore the object argument if there is one, since we don't have an object
    // type.
    Candidate.IgnoreObjectArgument =
        isa<CXXMethodDecl>(Candidate.Function) &&
        !isa<CXXConstructorDecl>(Candidate.Function);
    Candidate.ExplicitCallArguments = Args.size();
    if (Result == TDK_NonDependentConversionFailure)
      Candidate.FailureKind = ovl_fail_bad_conversion;
    else {
      Candidate.FailureKind = ovl_fail_bad_deduction;
      Candidate.DeductionFailure = MakeDeductionFailureInfo(Context, Result,
                                                            Info);
    }
    return;
  }

  // Add the function template specialization produced by template argument
  // deduction as a candidate.
  assert(Specialization && "Missing function template specialization?");
  AddOverloadCandidate(
      Specialization, FoundDecl, Args, CandidateSet, SuppressUserConversions,
      PartialOverloading, AllowExplicit,
      /*AllowExplicitConversions*/ false, IsADLCandidate, Conversions, PO);
}

/// Check that implicit conversion sequences can be formed for each argument
/// whose corresponding parameter has a non-dependent type, per DR1391's
/// [temp.deduct.call]p10.
bool Sema::CheckNonDependentConversions(
    FunctionTemplateDecl *FunctionTemplate, ArrayRef<QualType> ParamTypes,
    ArrayRef<Expr *> Args, OverloadCandidateSet &CandidateSet,
    ConversionSequenceList &Conversions, bool SuppressUserConversions,
    CXXRecordDecl *ActingContext, QualType ObjectType,
    Expr::Classification ObjectClassification, OverloadCandidateParamOrder PO) {
  // FIXME: The cases in which we allow explicit conversions for constructor
  // arguments never consider calling a constructor template. It's not clear
  // that is correct.
  const bool AllowExplicit = false;

  auto *FD = FunctionTemplate->getTemplatedDecl();
  auto *Method = dyn_cast<CXXMethodDecl>(FD);
  bool HasThisConversion = Method && !isa<CXXConstructorDecl>(Method);
  unsigned ThisConversions = HasThisConversion ? 1 : 0;

  Conversions =
      CandidateSet.allocateConversionSequences(ThisConversions + Args.size());

  // Overload resolution is always an unevaluated context.
  EnterExpressionEvaluationContext Unevaluated(
      *this, Sema::ExpressionEvaluationContext::Unevaluated);

  // For a method call, check the 'this' conversion here too. DR1391 doesn't
  // require that, but this check should never result in a hard error, and
  // overload resolution is permitted to sidestep instantiations.
  if (HasThisConversion && !cast<CXXMethodDecl>(FD)->isStatic() &&
      !ObjectType.isNull()) {
    unsigned ConvIdx = PO == OverloadCandidateParamOrder::Reversed ? 1 : 0;
    Conversions[ConvIdx] = TryObjectArgumentInitialization(
        *this, CandidateSet.getLocation(), ObjectType, ObjectClassification,
        Method, ActingContext);
    if (Conversions[ConvIdx].isBad())
      return true;
  }

  for (unsigned I = 0, N = std::min(ParamTypes.size(), Args.size()); I != N;
       ++I) {
    QualType ParamType = ParamTypes[I];
    if (!ParamType->isDependentType()) {
      unsigned ConvIdx = PO == OverloadCandidateParamOrder::Reversed
                             ? 0
                             : (ThisConversions + I);
      Conversions[ConvIdx]
        = TryCopyInitialization(*this, Args[I], ParamType,
                                SuppressUserConversions,
                                /*InOverloadResolution=*/true,
                                /*AllowObjCWritebackConversion=*/
                                  getLangOpts().ObjCAutoRefCount,
                                AllowExplicit);
      if (Conversions[ConvIdx].isBad())
        return true;
    }
  }

  return false;
}

/// Determine whether this is an allowable conversion from the result
/// of an explicit conversion operator to the expected type, per C++
/// [over.match.conv]p1 and [over.match.ref]p1.
///
/// \param ConvType The return type of the conversion function.
///
/// \param ToType The type we are converting to.
///
/// \param AllowObjCPointerConversion Allow a conversion from one
/// Objective-C pointer to another.
///
/// \returns true if the conversion is allowable, false otherwise.
static bool isAllowableExplicitConversion(Sema &S,
                                          QualType ConvType, QualType ToType,
                                          bool AllowObjCPointerConversion) {
  QualType ToNonRefType = ToType.getNonReferenceType();

  // Easy case: the types are the same.
  if (S.Context.hasSameUnqualifiedType(ConvType, ToNonRefType))
    return true;

  // Allow qualification conversions.
  bool ObjCLifetimeConversion;
  if (S.IsQualificationConversion(ConvType, ToNonRefType, /*CStyle*/false,
                                  ObjCLifetimeConversion))
    return true;

  // If we're not allowed to consider Objective-C pointer conversions,
  // we're done.
  if (!AllowObjCPointerConversion)
    return false;

  // Is this an Objective-C pointer conversion?
  bool IncompatibleObjC = false;
  QualType ConvertedType;
  return S.isObjCPointerConversion(ConvType, ToNonRefType, ConvertedType,
                                   IncompatibleObjC);
}

/// AddConversionCandidate - Add a C++ conversion function as a
/// candidate in the candidate set (C++ [over.match.conv],
/// C++ [over.match.copy]). From is the expression we're converting from,
/// and ToType is the type that we're eventually trying to convert to
/// (which may or may not be the same type as the type that the
/// conversion function produces).
void Sema::AddConversionCandidate(
    CXXConversionDecl *Conversion, DeclAccessPair FoundDecl,
    CXXRecordDecl *ActingContext, Expr *From, QualType ToType,
    OverloadCandidateSet &CandidateSet, bool AllowObjCConversionOnExplicit,
    bool AllowExplicit, bool AllowResultConversion) {
  assert(!Conversion->getDescribedFunctionTemplate() &&
         "Conversion function templates use AddTemplateConversionCandidate");
  QualType ConvType = Conversion->getConversionType().getNonReferenceType();
  if (!CandidateSet.isNewCandidate(Conversion))
    return;

  // If the conversion function has an undeduced return type, trigger its
  // deduction now.
  if (getLangOpts().CPlusPlus14 && ConvType->isUndeducedType()) {
    if (DeduceReturnType(Conversion, From->getExprLoc()))
      return;
    ConvType = Conversion->getConversionType().getNonReferenceType();
  }

  // If we don't allow any conversion of the result type, ignore conversion
  // functions that don't convert to exactly (possibly cv-qualified) T.
  if (!AllowResultConversion &&
      !Context.hasSameUnqualifiedType(Conversion->getConversionType(), ToType))
    return;

  // Per C++ [over.match.conv]p1, [over.match.ref]p1, an explicit conversion
  // operator is only a candidate if its return type is the target type or
  // can be converted to the target type with a qualification conversion.
  //
  // FIXME: Include such functions in the candidate list and explain why we
  // can't select them.
  if (Conversion->isExplicit() &&
      !isAllowableExplicitConversion(*this, ConvType, ToType,
                                     AllowObjCConversionOnExplicit))
    return;

  // Overload resolution is always an unevaluated context.
  EnterExpressionEvaluationContext Unevaluated(
      *this, Sema::ExpressionEvaluationContext::Unevaluated);

  // Add this candidate
  OverloadCandidate &Candidate = CandidateSet.addCandidate(1);
  Candidate.FoundDecl = FoundDecl;
  Candidate.Function = Conversion;
  Candidate.IsSurrogate = false;
  Candidate.IgnoreObjectArgument = false;
  Candidate.FinalConversion.setAsIdentityConversion();
  Candidate.FinalConversion.setFromType(ConvType);
  Candidate.FinalConversion.setAllToTypes(ToType);
  Candidate.Viable = true;
  Candidate.ExplicitCallArguments = 1;

  // Explicit functions are not actually candidates at all if we're not
  // allowing them in this context, but keep them around so we can point
  // to them in diagnostics.
  if (!AllowExplicit && Conversion->isExplicit()) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_explicit;
    return;
  }

  // C++ [over.match.funcs]p4:
  //   For conversion functions, the function is considered to be a member of
  //   the class of the implicit implied object argument for the purpose of
  //   defining the type of the implicit object parameter.
  //
  // Determine the implicit conversion sequence for the implicit
  // object parameter.
  QualType ImplicitParamType = From->getType();
  if (const PointerType *FromPtrType = ImplicitParamType->getAs<PointerType>())
    ImplicitParamType = FromPtrType->getPointeeType();
  CXXRecordDecl *ConversionContext
    = cast<CXXRecordDecl>(ImplicitParamType->castAs<RecordType>()->getDecl());

  Candidate.Conversions[0] = TryObjectArgumentInitialization(
      *this, CandidateSet.getLocation(), From->getType(),
      From->Classify(Context), Conversion, ConversionContext);

  if (Candidate.Conversions[0].isBad()) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_bad_conversion;
    return;
  }

  if (Conversion->getTrailingRequiresClause()) {
    ConstraintSatisfaction Satisfaction;
    if (CheckFunctionConstraints(Conversion, Satisfaction) ||
        !Satisfaction.IsSatisfied) {
      Candidate.Viable = false;
      Candidate.FailureKind = ovl_fail_constraints_not_satisfied;
      return;
    }
  }

  // We won't go through a user-defined type conversion function to convert a
  // derived to base as such conversions are given Conversion Rank. They only
  // go through a copy constructor. 13.3.3.1.2-p4 [over.ics.user]
  QualType FromCanon
    = Context.getCanonicalType(From->getType().getUnqualifiedType());
  QualType ToCanon = Context.getCanonicalType(ToType).getUnqualifiedType();
  if (FromCanon == ToCanon ||
      IsDerivedFrom(CandidateSet.getLocation(), FromCanon, ToCanon)) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_trivial_conversion;
    return;
  }

  // To determine what the conversion from the result of calling the
  // conversion function to the type we're eventually trying to
  // convert to (ToType), we need to synthesize a call to the
  // conversion function and attempt copy initialization from it. This
  // makes sure that we get the right semantics with respect to
  // lvalues/rvalues and the type. Fortunately, we can allocate this
  // call on the stack and we don't need its arguments to be
  // well-formed.
  DeclRefExpr ConversionRef(Context, Conversion, false, Conversion->getType(),
                            VK_LValue, From->getBeginLoc());
  ImplicitCastExpr ConversionFn(ImplicitCastExpr::OnStack,
                                Context.getPointerType(Conversion->getType()),
                                CK_FunctionToPointerDecay, &ConversionRef,
                                VK_PRValue, FPOptionsOverride());

  QualType ConversionType = Conversion->getConversionType();
  if (!isCompleteType(From->getBeginLoc(), ConversionType)) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_bad_final_conversion;
    return;
  }

  ExprValueKind VK = Expr::getValueKindForType(ConversionType);

  // Note that it is safe to allocate CallExpr on the stack here because
  // there are 0 arguments (i.e., nothing is allocated using ASTContext's
  // allocator).
  QualType CallResultType = ConversionType.getNonLValueExprType(Context);

  alignas(CallExpr) char Buffer[sizeof(CallExpr) + sizeof(Stmt *)];
  CallExpr *TheTemporaryCall = CallExpr::CreateTemporary(
      Buffer, &ConversionFn, CallResultType, VK, From->getBeginLoc());

  ImplicitConversionSequence ICS =
      TryCopyInitialization(*this, TheTemporaryCall, ToType,
                            /*SuppressUserConversions=*/true,
                            /*InOverloadResolution=*/false,
                            /*AllowObjCWritebackConversion=*/false);

  switch (ICS.getKind()) {
  case ImplicitConversionSequence::StandardConversion:
    Candidate.FinalConversion = ICS.Standard;

    // C++ [over.ics.user]p3:
    //   If the user-defined conversion is specified by a specialization of a
    //   conversion function template, the second standard conversion sequence
    //   shall have exact match rank.
    if (Conversion->getPrimaryTemplate() &&
        GetConversionRank(ICS.Standard.Second) != ICR_Exact_Match) {
      Candidate.Viable = false;
      Candidate.FailureKind = ovl_fail_final_conversion_not_exact;
      return;
    }

    // C++0x [dcl.init.ref]p5:
    //    In the second case, if the reference is an rvalue reference and
    //    the second standard conversion sequence of the user-defined
    //    conversion sequence includes an lvalue-to-rvalue conversion, the
    //    program is ill-formed.
    if (ToType->isRValueReferenceType() &&
        ICS.Standard.First == ICK_Lvalue_To_Rvalue) {
      Candidate.Viable = false;
      Candidate.FailureKind = ovl_fail_bad_final_conversion;
      return;
    }
    break;

  case ImplicitConversionSequence::BadConversion:
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_bad_final_conversion;
    return;

  default:
    llvm_unreachable(
           "Can only end up with a standard conversion sequence or failure");
  }

  if (EnableIfAttr *FailedAttr =
          CheckEnableIf(Conversion, CandidateSet.getLocation(), std::nullopt)) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_enable_if;
    Candidate.DeductionFailure.Data = FailedAttr;
    return;
  }

  if (Conversion->isMultiVersion() &&
      ((Conversion->hasAttr<TargetAttr>() &&
        !Conversion->getAttr<TargetAttr>()->isDefaultVersion()) ||
       (Conversion->hasAttr<TargetVersionAttr>() &&
        !Conversion->getAttr<TargetVersionAttr>()->isDefaultVersion()))) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_non_default_multiversion_function;
  }
}

/// Adds a conversion function template specialization
/// candidate to the overload set, using template argument deduction
/// to deduce the template arguments of the conversion function
/// template from the type that we are converting to (C++
/// [temp.deduct.conv]).
void Sema::AddTemplateConversionCandidate(
    FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
    CXXRecordDecl *ActingDC, Expr *From, QualType ToType,
    OverloadCandidateSet &CandidateSet, bool AllowObjCConversionOnExplicit,
    bool AllowExplicit, bool AllowResultConversion) {
  assert(isa<CXXConversionDecl>(FunctionTemplate->getTemplatedDecl()) &&
         "Only conversion function templates permitted here");

  if (!CandidateSet.isNewCandidate(FunctionTemplate))
    return;

  // If the function template has a non-dependent explicit specification,
  // exclude it now if appropriate; we are not permitted to perform deduction
  // and substitution in this case.
  if (!AllowExplicit && isNonDependentlyExplicit(FunctionTemplate)) {
    OverloadCandidate &Candidate = CandidateSet.addCandidate();
    Candidate.FoundDecl = FoundDecl;
    Candidate.Function = FunctionTemplate->getTemplatedDecl();
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_explicit;
    return;
  }

  TemplateDeductionInfo Info(CandidateSet.getLocation());
  CXXConversionDecl *Specialization = nullptr;
  if (TemplateDeductionResult Result
        = DeduceTemplateArguments(FunctionTemplate, ToType,
                                  Specialization, Info)) {
    OverloadCandidate &Candidate = CandidateSet.addCandidate();
    Candidate.FoundDecl = FoundDecl;
    Candidate.Function = FunctionTemplate->getTemplatedDecl();
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_bad_deduction;
    Candidate.IsSurrogate = false;
    Candidate.IgnoreObjectArgument = false;
    Candidate.ExplicitCallArguments = 1;
    Candidate.DeductionFailure = MakeDeductionFailureInfo(Context, Result,
                                                          Info);
    return;
  }

  // Add the conversion function template specialization produced by
  // template argument deduction as a candidate.
  assert(Specialization && "Missing function template specialization?");
  AddConversionCandidate(Specialization, FoundDecl, ActingDC, From, ToType,
                         CandidateSet, AllowObjCConversionOnExplicit,
                         AllowExplicit, AllowResultConversion);
}

/// AddSurrogateCandidate - Adds a "surrogate" candidate function that
/// converts the given @c Object to a function pointer via the
/// conversion function @c Conversion, and then attempts to call it
/// with the given arguments (C++ [over.call.object]p2-4). Proto is
/// the type of function that we'll eventually be calling.
void Sema::AddSurrogateCandidate(CXXConversionDecl *Conversion,
                                 DeclAccessPair FoundDecl,
                                 CXXRecordDecl *ActingContext,
                                 const FunctionProtoType *Proto,
                                 Expr *Object,
                                 ArrayRef<Expr *> Args,
                                 OverloadCandidateSet& CandidateSet) {
  if (!CandidateSet.isNewCandidate(Conversion))
    return;

  // Overload resolution is always an unevaluated context.
  EnterExpressionEvaluationContext Unevaluated(
      *this, Sema::ExpressionEvaluationContext::Unevaluated);

  OverloadCandidate &Candidate = CandidateSet.addCandidate(Args.size() + 1);
  Candidate.FoundDecl = FoundDecl;
  Candidate.Function = nullptr;
  Candidate.Surrogate = Conversion;
  Candidate.Viable = true;
  Candidate.IsSurrogate = true;
  Candidate.IgnoreObjectArgument = false;
  Candidate.ExplicitCallArguments = Args.size();

  // Determine the implicit conversion sequence for the implicit
  // object parameter.
  ImplicitConversionSequence ObjectInit = TryObjectArgumentInitialization(
      *this, CandidateSet.getLocation(), Object->getType(),
      Object->Classify(Context), Conversion, ActingContext);
  if (ObjectInit.isBad()) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_bad_conversion;
    Candidate.Conversions[0] = ObjectInit;
    return;
  }

  // The first conversion is actually a user-defined conversion whose
  // first conversion is ObjectInit's standard conversion (which is
  // effectively a reference binding). Record it as such.
  Candidate.Conversions[0].setUserDefined();
  Candidate.Conversions[0].UserDefined.Before = ObjectInit.Standard;
  Candidate.Conversions[0].UserDefined.EllipsisConversion = false;
  Candidate.Conversions[0].UserDefined.HadMultipleCandidates = false;
  Candidate.Conversions[0].UserDefined.ConversionFunction = Conversion;
  Candidate.Conversions[0].UserDefined.FoundConversionFunction = FoundDecl;
  Candidate.Conversions[0].UserDefined.After
    = Candidate.Conversions[0].UserDefined.Before;
  Candidate.Conversions[0].UserDefined.After.setAsIdentityConversion();

  // Find the
  unsigned NumParams = Proto->getNumParams();

  // (C++ 13.3.2p2): A candidate function having fewer than m
  // parameters is viable only if it has an ellipsis in its parameter
  // list (8.3.5).
  if (Args.size() > NumParams && !Proto->isVariadic()) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_too_many_arguments;
    return;
  }

  // Function types don't have any default arguments, so just check if
  // we have enough arguments.
  if (Args.size() < NumParams) {
    // Not enough arguments.
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_too_few_arguments;
    return;
  }

  // Determine the implicit conversion sequences for each of the
  // arguments.
  for (unsigned ArgIdx = 0, N = Args.size(); ArgIdx != N; ++ArgIdx) {
    if (ArgIdx < NumParams) {
      // (C++ 13.3.2p3): for F to be a viable function, there shall
      // exist for each argument an implicit conversion sequence
      // (13.3.3.1) that converts that argument to the corresponding
      // parameter of F.
      QualType ParamType = Proto->getParamType(ArgIdx);
      Candidate.Conversions[ArgIdx + 1]
        = TryCopyInitialization(*this, Args[ArgIdx], ParamType,
                                /*SuppressUserConversions=*/false,
                                /*InOverloadResolution=*/false,
                                /*AllowObjCWritebackConversion=*/
                                  getLangOpts().ObjCAutoRefCount);
      if (Candidate.Conversions[ArgIdx + 1].isBad()) {
        Candidate.Viable = false;
        Candidate.FailureKind = ovl_fail_bad_conversion;
        return;
      }
    } else {
      // (C++ 13.3.2p2): For the purposes of overload resolution, any
      // argument for which there is no corresponding parameter is
      // considered to ""match the ellipsis" (C+ 13.3.3.1.3).
      Candidate.Conversions[ArgIdx + 1].setEllipsis();
    }
  }

  if (EnableIfAttr *FailedAttr =
          CheckEnableIf(Conversion, CandidateSet.getLocation(), std::nullopt)) {
    Candidate.Viable = false;
    Candidate.FailureKind = ovl_fail_enable_if;
    Candidate.DeductionFailure.Data = FailedAttr;
    return;
  }
}

/// Add all of the non-member operator function declarations in the given
/// function set to the overload candidate set.
void Sema::AddNonMemberOperatorCandidates(
    const UnresolvedSetImpl &Fns, ArrayRef<Expr *> Args,
    OverloadCandidateSet &CandidateSet,
    TemplateArgumentListInfo *ExplicitTemplateArgs) {
  for (UnresolvedSetIterator F = Fns.begin(), E = Fns.end(); F != E; ++F) {
    NamedDecl *D = F.getDecl()->getUnderlyingDecl();
    ArrayRef<Expr *> FunctionArgs = Args;

    FunctionTemplateDecl *FunTmpl = dyn_cast<FunctionTemplateDecl>(D);
    FunctionDecl *FD =
        FunTmpl ? FunTmpl->getTemplatedDecl() : cast<FunctionDecl>(D);

    // Don't consider rewritten functions if we're not rewriting.
    if (!CandidateSet.getRewriteInfo().isAcceptableCandidate(FD))
      continue;

    assert(!isa<CXXMethodDecl>(FD) &&
           "unqualified operator lookup found a member function");

    if (FunTmpl) {
      AddTemplateOverloadCandidate(FunTmpl, F.getPair(), ExplicitTemplateArgs,
                                   FunctionArgs, CandidateSet);
      if (CandidateSet.getRewriteInfo().shouldAddReversed(*this, Args, FD))
        AddTemplateOverloadCandidate(
            FunTmpl, F.getPair(), ExplicitTemplateArgs,
            {FunctionArgs[1], FunctionArgs[0]}, CandidateSet, false, false,
            true, ADLCallKind::NotADL, OverloadCandidateParamOrder::Reversed);
    } else {
      if (ExplicitTemplateArgs)
        continue;
      AddOverloadCandidate(FD, F.getPair(), FunctionArgs, CandidateSet);
      if (CandidateSet.getRewriteInfo().shouldAddReversed(*this, Args, FD))
        AddOverloadCandidate(
            FD, F.getPair(), {FunctionArgs[1], FunctionArgs[0]}, CandidateSet,
            false, false, true, false, ADLCallKind::NotADL, std::nullopt,
            OverloadCandidateParamOrder::Reversed);
    }
  }
}

/// Add overload candidates for overloaded operators that are
/// member functions.
///
/// Add the overloaded operator candidates that are member functions
/// for the operator Op that was used in an operator expression such
/// as "x Op y". , Args/NumArgs provides the operator arguments, and
/// CandidateSet will store the added overload candidates. (C++
/// [over.match.oper]).
void Sema::AddMemberOperatorCandidates(OverloadedOperatorKind Op,
                                       SourceLocation OpLoc,
                                       ArrayRef<Expr *> Args,
                                       OverloadCandidateSet &CandidateSet,
                                       OverloadCandidateParamOrder PO) {
  DeclarationName OpName = Context.DeclarationNames.getCXXOperatorName(Op);

  // C++ [over.match.oper]p3:
  //   For a unary operator @ with an operand of a type whose
  //   cv-unqualified version is T1, and for a binary operator @ with
  //   a left operand of a type whose cv-unqualified version is T1 and
  //   a right operand of a type whose cv-unqualified version is T2,
  //   three sets of candidate functions, designated member
  //   candidates, non-member candidates and built-in candidates, are
  //   constructed as follows:
  QualType T1 = Args[0]->getType();

  //     -- If T1 is a complete class type or a class currently being
  //        defined, the set of member candidates is the result of the
  //        qualified lookup of T1::operator@ (13.3.1.1.1); otherwise,
  //        the set of member candidates is empty.
  if (const RecordType *T1Rec = T1->getAs<RecordType>()) {
    // Complete the type if it can be completed.
    if (!isCompleteType(OpLoc, T1) && !T1Rec->isBeingDefined())
      return;
    // If the type is neither complete nor being defined, bail out now.
    if (!T1Rec->getDecl()->getDefinition())
      return;

    LookupResult Operators(*this, OpName, OpLoc, LookupOrdinaryName);
    LookupQualifiedName(Operators, T1Rec->getDecl());
    Operators.suppressDiagnostics();

    for (LookupResult::iterator Oper = Operators.begin(),
                                OperEnd = Operators.end();
         Oper != OperEnd; ++Oper) {
      if (Oper->getAsFunction() &&
          PO == OverloadCandidateParamOrder::Reversed &&
          !CandidateSet.getRewriteInfo().shouldAddReversed(
              *this, {Args[1], Args[0]}, Oper->getAsFunction()))
        continue;
      AddMethodCandidate(Oper.getPair(), Args[0]->getType(),
                         Args[0]->Classify(Context), Args.slice(1),
                         CandidateSet, /*SuppressUserConversion=*/false, PO);
    }
  }
}

/// AddBuiltinCandidate - Add a candidate for a built-in
/// operator. ResultTy and ParamTys are the result and parameter types
/// of the built-in candidate, respectively. Args and NumArgs are the
/// arguments being passed to the candidate. IsAssignmentOperator
/// should be true when this built-in candidate is an assignment
/// operator. NumContextualBoolArguments is the number of arguments
/// (at the beginning of the argument list) that will be contextually
/// converted to bool.
void Sema::AddBuiltinCandidate(QualType *ParamTys, ArrayRef<Expr *> Args,
                               OverloadCandidateSet& CandidateSet,
                               bool IsAssignmentOperator,
                               unsigned NumContextualBoolArguments) {
  // Overload resolution is always an unevaluated context.
  EnterExpressionEvaluationContext Unevaluated(
      *this, Sema::ExpressionEvaluationContext::Unevaluated);

  // Add this candidate
  OverloadCandidate &Candidate = CandidateSet.addCandidate(Args.size());
  Candidate.FoundDecl = DeclAccessPair::make(nullptr, AS_none);
  Candidate.Function = nullptr;
  Candidate.IsSurrogate = false;
  Candidate.IgnoreObjectArgument = false;
  std::copy(ParamTys, ParamTys + Args.size(), Candidate.BuiltinParamTypes);

  // Determine the implicit conversion sequences for each of the
  // arguments.
  Candidate.Viable = true;
  Candidate.ExplicitCallArguments = Args.size();
  for (unsigned ArgIdx = 0, N = Args.size(); ArgIdx != N; ++ArgIdx) {
    // C++ [over.match.oper]p4:
    //   For the built-in assignment operators, conversions of the
    //   left operand are restricted as follows:
    //     -- no temporaries are introduced to hold the left operand, and
    //     -- no user-defined conversions are applied to the left
    //        operand to achieve a type match with the left-most
    //        parameter of a built-in candidate.
    //
    // We block these conversions by turning off user-defined
    // conversions, since that is the only way that initialization of
    // a reference to a non-class type can occur from something that
    // is not of the same type.
    if (ArgIdx < NumContextualBoolArguments) {
      assert(ParamTys[ArgIdx] == Context.BoolTy &&
             "Contextual conversion to bool requires bool type");
      Candidate.Conversions[ArgIdx]
        = TryContextuallyConvertToBool(*this, Args[ArgIdx]);
    } else {
      Candidate.Conversions[ArgIdx]
        = TryCopyInitialization(*this, Args[ArgIdx], ParamTys[ArgIdx],
                                ArgIdx == 0 && IsAssignmentOperator,
                                /*InOverloadResolution=*/false,
                                /*AllowObjCWritebackConversion=*/
                                  getLangOpts().ObjCAutoRefCount);
    }
    if (Candidate.Conversions[ArgIdx].isBad()) {
      Candidate.Viable = false;
      Candidate.FailureKind = ovl_fail_bad_conversion;
      break;
    }
  }
}

namespace {

/// BuiltinCandidateTypeSet - A set of types that will be used for the
/// candidate operator functions for built-in operators (C++
/// [over.built]). The types are separated into pointer types and
/// enumeration types.
class BuiltinCandidateTypeSet  {
  /// TypeSet - A set of types.
  typedef llvm::SetVector<QualType, SmallVector<QualType, 8>,
                          llvm::SmallPtrSet<QualType, 8>> TypeSet;

  /// PointerTypes - The set of pointer types that will be used in the
  /// built-in candidates.
  TypeSet PointerTypes;

  /// MemberPointerTypes - The set of member pointer types that will be
  /// used in the built-in candidates.
  TypeSet MemberPointerTypes;

  /// EnumerationTypes - The set of enumeration types that will be
  /// used in the built-in candidates.
  TypeSet EnumerationTypes;

  /// The set of vector types that will be used in the built-in
  /// candidates.
  TypeSet VectorTypes;

  /// The set of matrix types that will be used in the built-in
  /// candidates.
  TypeSet MatrixTypes;

  /// A flag indicating non-record types are viable candidates
  bool HasNonRecordTypes;

  /// A flag indicating whether either arithmetic or enumeration types
  /// were present in the candidate set.
  bool HasArithmeticOrEnumeralTypes;

  /// A flag indicating whether the nullptr type was present in the
  /// candidate set.
  bool HasNullPtrType;

  /// Sema - The semantic analysis instance where we are building the
  /// candidate type set.
  Sema &SemaRef;

  /// Context - The AST context in which we will build the type sets.
  ASTContext &Context;

  bool AddPointerWithMoreQualifiedTypeVariants(QualType Ty,
                                               const Qualifiers &VisibleQuals);
  bool AddMemberPointerWithMoreQualifiedTypeVariants(QualType Ty);

public:
  /// iterator - Iterates through the types that are part of the set.
  typedef TypeSet::iterator iterator;

  BuiltinCandidateTypeSet(Sema &SemaRef)
    : HasNonRecordTypes(false),
      HasArithmeticOrEnumeralTypes(false),
      HasNullPtrType(false),
      SemaRef(SemaRef),
      Context(SemaRef.Context) { }

  void AddTypesConvertedFrom(QualType Ty,
                             SourceLocation Loc,
                             bool AllowUserConversions,
                             bool AllowExplicitConversions,
                             const Qualifiers &VisibleTypeConversionsQuals);

  llvm::iterator_range<iterator> pointer_types() { return PointerTypes; }
  llvm::iterator_range<iterator> member_pointer_types() {
    return MemberPointerTypes;
  }
  llvm::iterator_range<iterator> enumeration_types() {
    return EnumerationTypes;
  }
  llvm::iterator_range<iterator> vector_types() { return VectorTypes; }
  llvm::iterator_range<iterator> matrix_types() { return MatrixTypes; }

  bool containsMatrixType(QualType Ty) const { return MatrixTypes.count(Ty); }
  bool hasNonRecordTypes() { return HasNonRecordTypes; }
  bool hasArithmeticOrEnumeralTypes() { return HasArithmeticOrEnumeralTypes; }
  bool hasNullPtrType() const { return HasNullPtrType; }
};

} // end anonymous namespace

/// AddPointerWithMoreQualifiedTypeVariants - Add the pointer type @p Ty to
/// the set of pointer types along with any more-qualified variants of
/// that type. For example, if @p Ty is "int const *", this routine
/// will add "int const *", "int const volatile *", "int const
/// restrict *", and "int const volatile restrict *" to the set of
/// pointer types. Returns true if the add of @p Ty itself succeeded,
/// false otherwise.
///
/// FIXME: what to do about extended qualifiers?
bool
BuiltinCandidateTypeSet::AddPointerWithMoreQualifiedTypeVariants(QualType Ty,
                                             const Qualifiers &VisibleQuals) {

  // Insert this type.
  if (!PointerTypes.insert(Ty))
    return false;

  QualType PointeeTy;
  const PointerType *PointerTy = Ty->getAs<PointerType>();
  bool buildObjCPtr = false;
  if (!PointerTy) {
    const ObjCObjectPointerType *PTy = Ty->castAs<ObjCObjectPointerType>();
    PointeeTy = PTy->getPointeeType();
    buildObjCPtr = true;
  } else {
    PointeeTy = PointerTy->getPointeeType();
  }

  // Don't add qualified variants of arrays. For one, they're not allowed
  // (the qualifier would sink to the element type), and for another, the
  // only overload situation where it matters is subscript or pointer +- int,
  // and those shouldn't have qualifier variants anyway.
  if (PointeeTy->isArrayType())
    return true;

  unsigned BaseCVR = PointeeTy.getCVRQualifiers();
  bool hasVolatile = VisibleQuals.hasVolatile();
  bool hasRestrict = VisibleQuals.hasRestrict();

  // Iterate through all strict supersets of BaseCVR.
  for (unsigned CVR = BaseCVR+1; CVR <= Qualifiers::CVRMask; ++CVR) {
    if ((CVR | BaseCVR) != CVR) continue;
    // Skip over volatile if no volatile found anywhere in the types.
    if ((CVR & Qualifiers::Volatile) && !hasVolatile) continue;

    // Skip over restrict if no restrict found anywhere in the types, or if
    // the type cannot be restrict-qualified.
    if ((CVR & Qualifiers::Restrict) &&
        (!hasRestrict ||
         (!(PointeeTy->isAnyPointerType() || PointeeTy->isReferenceType()))))
      continue;

    // Build qualified pointee type.
    QualType QPointeeTy = Context.getCVRQualifiedType(PointeeTy, CVR);

    // Build qualified pointer type.
    QualType QPointerTy;
    if (!buildObjCPtr)
      QPointerTy = Context.getPointerType(QPointeeTy);
    else
      QPointerTy = Context.getObjCObjectPointerType(QPointeeTy);

    // Insert qualified pointer type.
    PointerTypes.insert(QPointerTy);
  }

  return true;
}

/// AddMemberPointerWithMoreQualifiedTypeVariants - Add the pointer type @p Ty
/// to the set of pointer types along with any more-qualified variants of
/// that type. For example, if @p Ty is "int const *", this routine
/// will add "int const *", "int const volatile *", "int const
/// restrict *", and "int const volatile restrict *" to the set of
/// pointer types. Returns true if the add of @p Ty itself succeeded,
/// false otherwise.
///
/// FIXME: what to do about extended qualifiers?
bool
BuiltinCandidateTypeSet::AddMemberPointerWithMoreQualifiedTypeVariants(
    QualType Ty) {
  // Insert this type.
  if (!MemberPointerTypes.insert(Ty))
    return false;

  const MemberPointerType *PointerTy = Ty->getAs<MemberPointerType>();
  assert(PointerTy && "type was not a member pointer type!");

  QualType PointeeTy = PointerTy->getPointeeType();
  // Don't add qualified variants of arrays. For one, they're not allowed
  // (the qualifier would sink to the element type), and for another, the
  // only overload situation where it matters is subscript or pointer +- int,
  // and those shouldn't have qualifier variants anyway.
  if (PointeeTy->isArrayType())
    return true;
  const Type *ClassTy = PointerTy->getClass();

  // Iterate through all strict supersets of the pointee type's CVR
  // qualifiers.
  unsigned BaseCVR = PointeeTy.getCVRQualifiers();
  for (unsigned CVR = BaseCVR+1; CVR <= Qualifiers::CVRMask; ++CVR) {
    if ((CVR | BaseCVR) != CVR) continue;

    QualType QPointeeTy = Context.getCVRQualifiedType(PointeeTy, CVR);
    MemberPointerTypes.insert(
      Context.getMemberPointerType(QPointeeTy, ClassTy));
  }

  return true;
}

/// AddTypesConvertedFrom - Add each of the types to which the type @p
/// Ty can be implicit converted to the given set of @p Types. We're
/// primarily interested in pointer types and enumeration types. We also
/// take member pointer types, for the conditional operator.
/// AllowUserConversions is true if we should look at the conversion
/// functions of a class type, and AllowExplicitConversions if we
/// should also include the explicit conversion functions of a class
/// type.
void
BuiltinCandidateTypeSet::AddTypesConvertedFrom(QualType Ty,
                                               SourceLocation Loc,
                                               bool AllowUserConversions,
                                               bool AllowExplicitConversions,
                                               const Qualifiers &VisibleQuals) {
  // Only deal with canonical types.
  Ty = Context.getCanonicalType(Ty);

  // Look through reference types; they aren't part of the type of an
  // expression for the purposes of conversions.
  if (const ReferenceType *RefTy = Ty->getAs<ReferenceType>())
    Ty = RefTy->getPointeeType();

  // If we're dealing with an array type, decay to the pointer.
  if (Ty->isArrayType())
    Ty = SemaRef.Context.getArrayDecayedType(Ty);

  // Otherwise, we don't care about qualifiers on the type.
  Ty = Ty.getLocalUnqualifiedType();

  // Flag if we ever add a non-record type.
  const RecordType *TyRec = Ty->getAs<RecordType>();
  HasNonRecordTypes = HasNonRecordTypes || !TyRec;

  // Flag if we encounter an arithmetic type.
  HasArithmeticOrEnumeralTypes =
    HasArithmeticOrEnumeralTypes || Ty->isArithmeticType();

  if (Ty->isObjCIdType() || Ty->isObjCClassType())
    PointerTypes.insert(Ty);
  else if (Ty->getAs<PointerType>() || Ty->getAs<ObjCObjectPointerType>()) {
    // Insert our type, and its more-qualified variants, into the set
    // of types.
    if (!AddPointerWithMoreQualifiedTypeVariants(Ty, VisibleQuals))
      return;
  } else if (Ty->isMemberPointerType()) {
    // Member pointers are far easier, since the pointee can't be converted.
    if (!AddMemberPointerWithMoreQualifiedTypeVariants(Ty))
      return;
  } else if (Ty->isEnumeralType()) {
    HasArithmeticOrEnumeralTypes = true;
    EnumerationTypes.insert(Ty);
  } else if (Ty->isVectorType()) {
    // We treat vector types as arithmetic types in many contexts as an
    // extension.
    HasArithmeticOrEnumeralTypes = true;
    VectorTypes.insert(Ty);
  } else if (Ty->isMatrixType()) {
    // Similar to vector types, we treat vector types as arithmetic types in
    // many contexts as an extension.
    HasArithmeticOrEnumeralTypes = true;
    MatrixTypes.insert(Ty);
  } else if (Ty->isNullPtrType()) {
    HasNullPtrType = true;
  } else if (AllowUserConversions && TyRec) {
    // No conversion functions in incomplete types.
    if (!SemaRef.isCompleteType(Loc, Ty))
      return;

    CXXRecordDecl *ClassDecl = cast<CXXRecordDecl>(TyRec->getDecl());
    for (NamedDecl *D : ClassDecl->getVisibleConversionFunctions()) {
      if (isa<UsingShadowDecl>(D))
        D = cast<UsingShadowDecl>(D)->getTargetDecl();

      // Skip conversion function templates; they don't tell us anything
      // about which builtin types we can convert to.
      if (isa<FunctionTemplateDecl>(D))
        continue;

      CXXConversionDecl *Conv = cast<CXXConversionDecl>(D);
      if (AllowExplicitConversions || !Conv->isExplicit()) {
        AddTypesConvertedFrom(Conv->getConversionType(), Loc, false, false,
                              VisibleQuals);
      }
    }
  }
}
/// Helper function for adjusting address spaces for the pointer or reference
/// operands of builtin operators depending on the argument.
static QualType AdjustAddressSpaceForBuiltinOperandType(Sema &S, QualType T,
                                                        Expr *Arg) {
  return S.Context.getAddrSpaceQualType(T, Arg->getType().getAddressSpace());
}

/// Helper function for AddBuiltinOperatorCandidates() that adds
/// the volatile- and non-volatile-qualified assignment operators for the
/// given type to the candidate set.
static void AddBuiltinAssignmentOperatorCandidates(Sema &S,
                                                   QualType T,
                                                   ArrayRef<Expr *> Args,
                                    OverloadCandidateSet &CandidateSet) {
  QualType ParamTypes[2];

  // T& operator=(T&, T)
  ParamTypes[0] = S.Context.getLValueReferenceType(
      AdjustAddressSpaceForBuiltinOperandType(S, T, Args[0]));
  ParamTypes[1] = T;
  S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                        /*IsAssignmentOperator=*/true);

  if (!S.Context.getCanonicalType(T).isVolatileQualified()) {
    // volatile T& operator=(volatile T&, T)
    ParamTypes[0] = S.Context.getLValueReferenceType(
        AdjustAddressSpaceForBuiltinOperandType(S, S.Context.getVolatileType(T),
                                                Args[0]));
    ParamTypes[1] = T;
    S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                          /*IsAssignmentOperator=*/true);
  }
}

/// CollectVRQualifiers - This routine returns Volatile/Restrict qualifiers,
/// if any, found in visible type conversion functions found in ArgExpr's type.
static  Qualifiers CollectVRQualifiers(ASTContext &Context, Expr* ArgExpr) {
    Qualifiers VRQuals;
    const RecordType *TyRec;
    if (const MemberPointerType *RHSMPType =
        ArgExpr->getType()->getAs<MemberPointerType>())
      TyRec = RHSMPType->getClass()->getAs<RecordType>();
    else
      TyRec = ArgExpr->getType()->getAs<RecordType>();
    if (!TyRec) {
      // Just to be safe, assume the worst case.
      VRQuals.addVolatile();
      VRQuals.addRestrict();
      return VRQuals;
    }

    CXXRecordDecl *ClassDecl = cast<CXXRecordDecl>(TyRec->getDecl());
    if (!ClassDecl->hasDefinition())
      return VRQuals;

    for (NamedDecl *D : ClassDecl->getVisibleConversionFunctions()) {
      if (isa<UsingShadowDecl>(D))
        D = cast<UsingShadowDecl>(D)->getTargetDecl();
      if (CXXConversionDecl *Conv = dyn_cast<CXXConversionDecl>(D)) {
        QualType CanTy = Context.getCanonicalType(Conv->getConversionType());
        if (const ReferenceType *ResTypeRef = CanTy->getAs<ReferenceType>())
          CanTy = ResTypeRef->getPointeeType();
        // Need to go down the pointer/mempointer chain and add qualifiers
        // as see them.
        bool done = false;
        while (!done) {
          if (CanTy.isRestrictQualified())
            VRQuals.addRestrict();
          if (const PointerType *ResTypePtr = CanTy->getAs<PointerType>())
            CanTy = ResTypePtr->getPointeeType();
          else if (const MemberPointerType *ResTypeMPtr =
                CanTy->getAs<MemberPointerType>())
            CanTy = ResTypeMPtr->getPointeeType();
          else
            done = true;
          if (CanTy.isVolatileQualified())
            VRQuals.addVolatile();
          if (VRQuals.hasRestrict() && VRQuals.hasVolatile())
            return VRQuals;
        }
      }
    }
    return VRQuals;
}

// Note: We're currently only handling qualifiers that are meaningful for the
// LHS of compound assignment overloading.
static void forAllQualifierCombinationsImpl(
    QualifiersAndAtomic Available, QualifiersAndAtomic Applied,
    llvm::function_ref<void(QualifiersAndAtomic)> Callback) {
  // _Atomic
  if (Available.hasAtomic()) {
    Available.removeAtomic();
    forAllQualifierCombinationsImpl(Available, Applied.withAtomic(), Callback);
    forAllQualifierCombinationsImpl(Available, Applied, Callback);
    return;
  }

  // volatile
  if (Available.hasVolatile()) {
    Available.removeVolatile();
    assert(!Applied.hasVolatile());
    forAllQualifierCombinationsImpl(Available, Applied.withVolatile(),
                                    Callback);
    forAllQualifierCombinationsImpl(Available, Applied, Callback);
    return;
  }

  Callback(Applied);
}

static void forAllQualifierCombinations(
    QualifiersAndAtomic Quals,
    llvm::function_ref<void(QualifiersAndAtomic)> Callback) {
  return forAllQualifierCombinationsImpl(Quals, QualifiersAndAtomic(),
                                         Callback);
}

static QualType makeQualifiedLValueReferenceType(QualType Base,
                                                 QualifiersAndAtomic Quals,
                                                 Sema &S) {
  if (Quals.hasAtomic())
    Base = S.Context.getAtomicType(Base);
  if (Quals.hasVolatile())
    Base = S.Context.getVolatileType(Base);
  return S.Context.getLValueReferenceType(Base);
}

namespace {

/// Helper class to manage the addition of builtin operator overload
/// candidates. It provides shared state and utility methods used throughout
/// the process, as well as a helper method to add each group of builtin
/// operator overloads from the standard to a candidate set.
class BuiltinOperatorOverloadBuilder {
  // Common instance state available to all overload candidate addition methods.
  Sema &S;
  ArrayRef<Expr *> Args;
  QualifiersAndAtomic VisibleTypeConversionsQuals;
  bool HasArithmeticOrEnumeralCandidateType;
  SmallVectorImpl<BuiltinCandidateTypeSet> &CandidateTypes;
  OverloadCandidateSet &CandidateSet;

  static constexpr int ArithmeticTypesCap = 24;
  SmallVector<CanQualType, ArithmeticTypesCap> ArithmeticTypes;

  // Define some indices used to iterate over the arithmetic types in
  // ArithmeticTypes.  The "promoted arithmetic types" are the arithmetic
  // types are that preserved by promotion (C++ [over.built]p2).
  unsigned FirstIntegralType,
           LastIntegralType;
  unsigned FirstPromotedIntegralType,
           LastPromotedIntegralType;
  unsigned FirstPromotedArithmeticType,
           LastPromotedArithmeticType;
  unsigned NumArithmeticTypes;

  void InitArithmeticTypes() {
    // Start of promoted types.
    FirstPromotedArithmeticType = 0;
    ArithmeticTypes.push_back(S.Context.FloatTy);
    ArithmeticTypes.push_back(S.Context.DoubleTy);
    ArithmeticTypes.push_back(S.Context.LongDoubleTy);
    if (S.Context.getTargetInfo().hasFloat128Type())
      ArithmeticTypes.push_back(S.Context.Float128Ty);
    if (S.Context.getTargetInfo().hasIbm128Type())
      ArithmeticTypes.push_back(S.Context.Ibm128Ty);

    // Start of integral types.
    FirstIntegralType = ArithmeticTypes.size();
    FirstPromotedIntegralType = ArithmeticTypes.size();
    ArithmeticTypes.push_back(S.Context.IntTy);
    ArithmeticTypes.push_back(S.Context.LongTy);
    ArithmeticTypes.push_back(S.Context.LongLongTy);
    if (S.Context.getTargetInfo().hasInt128Type() ||
        (S.Context.getAuxTargetInfo() &&
         S.Context.getAuxTargetInfo()->hasInt128Type()))
      ArithmeticTypes.push_back(S.Context.Int128Ty);
    ArithmeticTypes.push_back(S.Context.UnsignedIntTy);
    ArithmeticTypes.push_back(S.Context.UnsignedLongTy);
    ArithmeticTypes.push_back(S.Context.UnsignedLongLongTy);
    if (S.Context.getTargetInfo().hasInt128Type() ||
        (S.Context.getAuxTargetInfo() &&
         S.Context.getAuxTargetInfo()->hasInt128Type()))
      ArithmeticTypes.push_back(S.Context.UnsignedInt128Ty);
    LastPromotedIntegralType = ArithmeticTypes.size();
    LastPromotedArithmeticType = ArithmeticTypes.size();
    // End of promoted types.

    ArithmeticTypes.push_back(S.Context.BoolTy);
    ArithmeticTypes.push_back(S.Context.CharTy);
    ArithmeticTypes.push_back(S.Context.WCharTy);
    if (S.Context.getLangOpts().Char8)
      ArithmeticTypes.push_back(S.Context.Char8Ty);
    ArithmeticTypes.push_back(S.Context.Char16Ty);
    ArithmeticTypes.push_back(S.Context.Char32Ty);
    ArithmeticTypes.push_back(S.Context.SignedCharTy);
    ArithmeticTypes.push_back(S.Context.ShortTy);
    ArithmeticTypes.push_back(S.Context.UnsignedCharTy);
    ArithmeticTypes.push_back(S.Context.UnsignedShortTy);
    LastIntegralType = ArithmeticTypes.size();
    NumArithmeticTypes = ArithmeticTypes.size();
    // End of integral types.
    // FIXME: What about complex? What about half?

    assert(ArithmeticTypes.size() <= ArithmeticTypesCap &&
           "Enough inline storage for all arithmetic types.");
  }

  /// Helper method to factor out the common pattern of adding overloads
  /// for '++' and '--' builtin operators.
  void addPlusPlusMinusMinusStyleOverloads(QualType CandidateTy,
                                           bool HasVolatile,
                                           bool HasRestrict) {
    QualType ParamTypes[2] = {
      S.Context.getLValueReferenceType(CandidateTy),
      S.Context.IntTy
    };

    // Non-volatile version.
    S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);

    // Use a heuristic to reduce number of builtin candidates in the set:
    // add volatile version only if there are conversions to a volatile type.
    if (HasVolatile) {
      ParamTypes[0] =
        S.Context.getLValueReferenceType(
          S.Context.getVolatileType(CandidateTy));
      S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
    }

    // Add restrict version only if there are conversions to a restrict type
    // and our candidate type is a non-restrict-qualified pointer.
    if (HasRestrict && CandidateTy->isAnyPointerType() &&
        !CandidateTy.isRestrictQualified()) {
      ParamTypes[0]
        = S.Context.getLValueReferenceType(
            S.Context.getCVRQualifiedType(CandidateTy, Qualifiers::Restrict));
      S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);

      if (HasVolatile) {
        ParamTypes[0]
          = S.Context.getLValueReferenceType(
              S.Context.getCVRQualifiedType(CandidateTy,
                                            (Qualifiers::Volatile |
                                             Qualifiers::Restrict)));
        S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
      }
    }

  }

  /// Helper to add an overload candidate for a binary builtin with types \p L
  /// and \p R.
  void AddCandidate(QualType L, QualType R) {
    QualType LandR[2] = {L, R};
    S.AddBuiltinCandidate(LandR, Args, CandidateSet);
  }

public:
  BuiltinOperatorOverloadBuilder(
    Sema &S, ArrayRef<Expr *> Args,
    QualifiersAndAtomic VisibleTypeConversionsQuals,
    bool HasArithmeticOrEnumeralCandidateType,
    SmallVectorImpl<BuiltinCandidateTypeSet> &CandidateTypes,
    OverloadCandidateSet &CandidateSet)
    : S(S), Args(Args),
      VisibleTypeConversionsQuals(VisibleTypeConversionsQuals),
      HasArithmeticOrEnumeralCandidateType(
        HasArithmeticOrEnumeralCandidateType),
      CandidateTypes(CandidateTypes),
      CandidateSet(CandidateSet) {

    InitArithmeticTypes();
  }

  // Increment is deprecated for bool since C++17.
  //
  // C++ [over.built]p3:
  //
  //   For every pair (T, VQ), where T is an arithmetic type other
  //   than bool, and VQ is either volatile or empty, there exist
  //   candidate operator functions of the form
  //
  //       VQ T&      operator++(VQ T&);
  //       T          operator++(VQ T&, int);
  //
  // C++ [over.built]p4:
  //
  //   For every pair (T, VQ), where T is an arithmetic type other
  //   than bool, and VQ is either volatile or empty, there exist
  //   candidate operator functions of the form
  //
  //       VQ T&      operator--(VQ T&);
  //       T          operator--(VQ T&, int);
  void addPlusPlusMinusMinusArithmeticOverloads(OverloadedOperatorKind Op) {
    if (!HasArithmeticOrEnumeralCandidateType)
      return;

    for (unsigned Arith = 0; Arith < NumArithmeticTypes; ++Arith) {
      const auto TypeOfT = ArithmeticTypes[Arith];
      if (TypeOfT == S.Context.BoolTy) {
        if (Op == OO_MinusMinus)
          continue;
        if (Op == OO_PlusPlus && S.getLangOpts().CPlusPlus17)
          continue;
      }
      addPlusPlusMinusMinusStyleOverloads(
        TypeOfT,
        VisibleTypeConversionsQuals.hasVolatile(),
        VisibleTypeConversionsQuals.hasRestrict());
    }
  }

  // C++ [over.built]p5:
  //
  //   For every pair (T, VQ), where T is a cv-qualified or
  //   cv-unqualified object type, and VQ is either volatile or
  //   empty, there exist candidate operator functions of the form
  //
  //       T*VQ&      operator++(T*VQ&);
  //       T*VQ&      operator--(T*VQ&);
  //       T*         operator++(T*VQ&, int);
  //       T*         operator--(T*VQ&, int);
  void addPlusPlusMinusMinusPointerOverloads() {
    for (QualType PtrTy : CandidateTypes[0].pointer_types()) {
      // Skip pointer types that aren't pointers to object types.
      if (!PtrTy->getPointeeType()->isObjectType())
        continue;

      addPlusPlusMinusMinusStyleOverloads(
          PtrTy,
          (!PtrTy.isVolatileQualified() &&
           VisibleTypeConversionsQuals.hasVolatile()),
          (!PtrTy.isRestrictQualified() &&
           VisibleTypeConversionsQuals.hasRestrict()));
    }
  }

  // C++ [over.built]p6:
  //   For every cv-qualified or cv-unqualified object type T, there
  //   exist candidate operator functions of the form
  //
  //       T&         operator*(T*);
  //
  // C++ [over.built]p7:
  //   For every function type T that does not have cv-qualifiers or a
  //   ref-qualifier, there exist candidate operator functions of the form
  //       T&         operator*(T*);
  void addUnaryStarPointerOverloads() {
    for (QualType ParamTy : CandidateTypes[0].pointer_types()) {
      QualType PointeeTy = ParamTy->getPointeeType();
      if (!PointeeTy->isObjectType() && !PointeeTy->isFunctionType())
        continue;

      if (const FunctionProtoType *Proto =PointeeTy->getAs<FunctionProtoType>())
        if (Proto->getMethodQuals() || Proto->getRefQualifier())
          continue;

      S.AddBuiltinCandidate(&ParamTy, Args, CandidateSet);
    }
  }

  // C++ [over.built]p9:
  //  For every promoted arithmetic type T, there exist candidate
  //  operator functions of the form
  //
  //       T         operator+(T);
  //       T         operator-(T);
  void addUnaryPlusOrMinusArithmeticOverloads() {
    if (!HasArithmeticOrEnumeralCandidateType)
      return;

    for (unsigned Arith = FirstPromotedArithmeticType;
         Arith < LastPromotedArithmeticType; ++Arith) {
      QualType ArithTy = ArithmeticTypes[Arith];
      S.AddBuiltinCandidate(&ArithTy, Args, CandidateSet);
    }

    // Extension: We also add these operators for vector types.
    for (QualType VecTy : CandidateTypes[0].vector_types())
      S.AddBuiltinCandidate(&VecTy, Args, CandidateSet);
  }

  // C++ [over.built]p8:
  //   For every type T, there exist candidate operator functions of
  //   the form
  //
  //       T*         operator+(T*);
  void addUnaryPlusPointerOverloads() {
    for (QualType ParamTy : CandidateTypes[0].pointer_types())
      S.AddBuiltinCandidate(&ParamTy, Args, CandidateSet);
  }

  // C++ [over.built]p10:
  //   For every promoted integral type T, there exist candidate
  //   operator functions of the form
  //
  //        T         operator~(T);
  void addUnaryTildePromotedIntegralOverloads() {
    if (!HasArithmeticOrEnumeralCandidateType)
      return;

    for (unsigned Int = FirstPromotedIntegralType;
         Int < LastPromotedIntegralType; ++Int) {
      QualType IntTy = ArithmeticTypes[Int];
      S.AddBuiltinCandidate(&IntTy, Args, CandidateSet);
    }

    // Extension: We also add this operator for vector types.
    for (QualType VecTy : CandidateTypes[0].vector_types())
      S.AddBuiltinCandidate(&VecTy, Args, CandidateSet);
  }

  // C++ [over.match.oper]p16:
  //   For every pointer to member type T or type std::nullptr_t, there
  //   exist candidate operator functions of the form
  //
  //        bool operator==(T,T);
  //        bool operator!=(T,T);
  void addEqualEqualOrNotEqualMemberPointerOrNullptrOverloads() {
    /// Set of (canonical) types that we've already handled.
    llvm::SmallPtrSet<QualType, 8> AddedTypes;

    for (unsigned ArgIdx = 0, N = Args.size(); ArgIdx != N; ++ArgIdx) {
      for (QualType MemPtrTy : CandidateTypes[ArgIdx].member_pointer_types()) {
        // Don't add the same builtin candidate twice.
        if (!AddedTypes.insert(S.Context.getCanonicalType(MemPtrTy)).second)
          continue;

        QualType ParamTypes[2] = {MemPtrTy, MemPtrTy};
        S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
      }

      if (CandidateTypes[ArgIdx].hasNullPtrType()) {
        CanQualType NullPtrTy = S.Context.getCanonicalType(S.Context.NullPtrTy);
        if (AddedTypes.insert(NullPtrTy).second) {
          QualType ParamTypes[2] = { NullPtrTy, NullPtrTy };
          S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
        }
      }
    }
  }

  // C++ [over.built]p15:
  //
  //   For every T, where T is an enumeration type or a pointer type,
  //   there exist candidate operator functions of the form
  //
  //        bool       operator<(T, T);
  //        bool       operator>(T, T);
  //        bool       operator<=(T, T);
  //        bool       operator>=(T, T);
  //        bool       operator==(T, T);
  //        bool       operator!=(T, T);
  //           R       operator<=>(T, T)
  void addGenericBinaryPointerOrEnumeralOverloads(bool IsSpaceship) {
    // C++ [over.match.oper]p3:
    //   [...]the built-in candidates include all of the candidate operator
    //   functions defined in 13.6 that, compared to the given operator, [...]
    //   do not have the same parameter-type-list as any non-template non-member
    //   candidate.
    //
    // Note that in practice, this only affects enumeration types because there
    // aren't any built-in candidates of record type, and a user-defined operator
    // must have an operand of record or enumeration type. Also, the only other
    // overloaded operator with enumeration arguments, operator=,
    // cannot be overloaded for enumeration types, so this is the only place
    // where we must suppress candidates like this.
    llvm::DenseSet<std::pair<CanQualType, CanQualType> >
      UserDefinedBinaryOperators;

    for (unsigned ArgIdx = 0, N = Args.size(); ArgIdx != N; ++ArgIdx) {
      if (!CandidateTypes[ArgIdx].enumeration_types().empty()) {
        for (OverloadCandidateSet::iterator C = CandidateSet.begin(),
                                         CEnd = CandidateSet.end();
             C != CEnd; ++C) {
          if (!C->Viable || !C->Function || C->Function->getNumParams() != 2)
            continue;

          if (C->Function->isFunctionTemplateSpecialization())
            continue;

          // We interpret "same parameter-type-list" as applying to the
          // "synthesized candidate, with the order of the two parameters
          // reversed", not to the original function.
          bool Reversed = C->isReversed();
          QualType FirstParamType = C->Function->getParamDecl(Reversed ? 1 : 0)
                                        ->getType()
                                        .getUnqualifiedType();
          QualType SecondParamType = C->Function->getParamDecl(Reversed ? 0 : 1)
                                         ->getType()
                                         .getUnqualifiedType();

          // Skip if either parameter isn't of enumeral type.
          if (!FirstParamType->isEnumeralType() ||
              !SecondParamType->isEnumeralType())
            continue;

          // Add this operator to the set of known user-defined operators.
          UserDefinedBinaryOperators.insert(
            std::make_pair(S.Context.getCanonicalType(FirstParamType),
                           S.Context.getCanonicalType(SecondParamType)));
        }
      }
    }

    /// Set of (canonical) types that we've already handled.
    llvm::SmallPtrSet<QualType, 8> AddedTypes;

    for (unsigned ArgIdx = 0, N = Args.size(); ArgIdx != N; ++ArgIdx) {
      for (QualType PtrTy : CandidateTypes[ArgIdx].pointer_types()) {
        // Don't add the same builtin candidate twice.
        if (!AddedTypes.insert(S.Context.getCanonicalType(PtrTy)).second)
          continue;
        if (IsSpaceship && PtrTy->isFunctionPointerType())
          continue;

        QualType ParamTypes[2] = {PtrTy, PtrTy};
        S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
      }
      for (QualType EnumTy : CandidateTypes[ArgIdx].enumeration_types()) {
        CanQualType CanonType = S.Context.getCanonicalType(EnumTy);

        // Don't add the same builtin candidate twice, or if a user defined
        // candidate exists.
        if (!AddedTypes.insert(CanonType).second ||
            UserDefinedBinaryOperators.count(std::make_pair(CanonType,
                                                            CanonType)))
          continue;
        QualType ParamTypes[2] = {EnumTy, EnumTy};
        S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
      }
    }
  }

  // C++ [over.built]p13:
  //
  //   For every cv-qualified or cv-unqualified object type T
  //   there exist candidate operator functions of the form
  //
  //      T*         operator+(T*, ptrdiff_t);
  //      T&         operator[](T*, ptrdiff_t);    [BELOW]
  //      T*         operator-(T*, ptrdiff_t);
  //      T*         operator+(ptrdiff_t, T*);
  //      T&         operator[](ptrdiff_t, T*);    [BELOW]
  //
  // C++ [over.built]p14:
  //
  //   For every T, where T is a pointer to object type, there
  //   exist candidate operator functions of the form
  //
  //      ptrdiff_t  operator-(T, T);
  void addBinaryPlusOrMinusPointerOverloads(OverloadedOperatorKind Op) {
    /// Set of (canonical) types that we've already handled.
    llvm::SmallPtrSet<QualType, 8> AddedTypes;

    for (int Arg = 0; Arg < 2; ++Arg) {
      QualType AsymmetricParamTypes[2] = {
        S.Context.getPointerDiffType(),
        S.Context.getPointerDiffType(),
      };
      for (QualType PtrTy : CandidateTypes[Arg].pointer_types()) {
        QualType PointeeTy = PtrTy->getPointeeType();
        if (!PointeeTy->isObjectType())
          continue;

        AsymmetricParamTypes[Arg] = PtrTy;
        if (Arg == 0 || Op == OO_Plus) {
          // operator+(T*, ptrdiff_t) or operator-(T*, ptrdiff_t)
          // T* operator+(ptrdiff_t, T*);
          S.AddBuiltinCandidate(AsymmetricParamTypes, Args, CandidateSet);
        }
        if (Op == OO_Minus) {
          // ptrdiff_t operator-(T, T);
          if (!AddedTypes.insert(S.Context.getCanonicalType(PtrTy)).second)
            continue;

          QualType ParamTypes[2] = {PtrTy, PtrTy};
          S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
        }
      }
    }
  }

  // C++ [over.built]p12:
  //
  //   For every pair of promoted arithmetic types L and R, there
  //   exist candidate operator functions of the form
  //
  //        LR         operator*(L, R);
  //        LR         operator/(L, R);
  //        LR         operator+(L, R);
  //        LR         operator-(L, R);
  //        bool       operator<(L, R);
  //        bool       operator>(L, R);
  //        bool       operator<=(L, R);
  //        bool       operator>=(L, R);
  //        bool       operator==(L, R);
  //        bool       operator!=(L, R);
  //
  //   where LR is the result of the usual arithmetic conversions
  //   between types L and R.
  //
  // C++ [over.built]p24:
  //
  //   For every pair of promoted arithmetic types L and R, there exist
  //   candidate operator functions of the form
  //
  //        LR       operator?(bool, L, R);
  //
  //   where LR is the result of the usual arithmetic conversions
  //   between types L and R.
  // Our candidates ignore the first parameter.
  void addGenericBinaryArithmeticOverloads() {
    if (!HasArithmeticOrEnumeralCandidateType)
      return;

    for (unsigned Left = FirstPromotedArithmeticType;
         Left < LastPromotedArithmeticType; ++Left) {
      for (unsigned Right = FirstPromotedArithmeticType;
           Right < LastPromotedArithmeticType; ++Right) {
        QualType LandR[2] = { ArithmeticTypes[Left],
                              ArithmeticTypes[Right] };
        S.AddBuiltinCandidate(LandR, Args, CandidateSet);
      }
    }

    // Extension: Add the binary operators ==, !=, <, <=, >=, >, *, /, and the
    // conditional operator for vector types.
    for (QualType Vec1Ty : CandidateTypes[0].vector_types())
      for (QualType Vec2Ty : CandidateTypes[1].vector_types()) {
        QualType LandR[2] = {Vec1Ty, Vec2Ty};
        S.AddBuiltinCandidate(LandR, Args, CandidateSet);
      }
  }

  /// Add binary operator overloads for each candidate matrix type M1, M2:
  ///  * (M1, M1) -> M1
  ///  * (M1, M1.getElementType()) -> M1
  ///  * (M2.getElementType(), M2) -> M2
  ///  * (M2, M2) -> M2 // Only if M2 is not part of CandidateTypes[0].
  void addMatrixBinaryArithmeticOverloads() {
    if (!HasArithmeticOrEnumeralCandidateType)
      return;

    for (QualType M1 : CandidateTypes[0].matrix_types()) {
      AddCandidate(M1, cast<MatrixType>(M1)->getElementType());
      AddCandidate(M1, M1);
    }

    for (QualType M2 : CandidateTypes[1].matrix_types()) {
      AddCandidate(cast<MatrixType>(M2)->getElementType(), M2);
      if (!CandidateTypes[0].containsMatrixType(M2))
        AddCandidate(M2, M2);
    }
  }

  // C++2a [over.built]p14:
  //
  //   For every integral type T there exists a candidate operator function
  //   of the form
  //
  //        std::strong_ordering operator<=>(T, T)
  //
  // C++2a [over.built]p15:
  //
  //   For every pair of floating-point types L and R, there exists a candidate
  //   operator function of the form
  //
  //       std::partial_ordering operator<=>(L, R);
  //
  // FIXME: The current specification for integral types doesn't play nice with
  // the direction of p0946r0, which allows mixed integral and unscoped-enum
  // comparisons. Under the current spec this can lead to ambiguity during
  // overload resolution. For example:
  //
  //   enum A : int {a};
  //   auto x = (a <=> (long)42);
  //
  //   error: call is ambiguous for arguments 'A' and 'long'.
  //   note: candidate operator<=>(int, int)
  //   note: candidate operator<=>(long, long)
  //
  // To avoid this error, this function deviates from the specification and adds
  // the mixed overloads `operator<=>(L, R)` where L and R are promoted
  // arithmetic types (the same as the generic relational overloads).
  //
  // For now this function acts as a placeholder.
  void addThreeWayArithmeticOverloads() {
    addGenericBinaryArithmeticOverloads();
  }

  // C++ [over.built]p17:
  //
  //   For every pair of promoted integral types L and R, there
  //   exist candidate operator functions of the form
  //
  //      LR         operator%(L, R);
  //      LR         operator&(L, R);
  //      LR         operator^(L, R);
  //      LR         operator|(L, R);
  //      L          operator<<(L, R);
  //      L          operator>>(L, R);
  //
  //   where LR is the result of the usual arithmetic conversions
  //   between types L and R.
  void addBinaryBitwiseArithmeticOverloads() {
    if (!HasArithmeticOrEnumeralCandidateType)
      return;

    for (unsigned Left = FirstPromotedIntegralType;
         Left < LastPromotedIntegralType; ++Left) {
      for (unsigned Right = FirstPromotedIntegralType;
           Right < LastPromotedIntegralType; ++Right) {
        QualType LandR[2] = { ArithmeticTypes[Left],
                              ArithmeticTypes[Right] };
        S.AddBuiltinCandidate(LandR, Args, CandidateSet);
      }
    }
  }

  // C++ [over.built]p20:
  //
  //   For every pair (T, VQ), where T is an enumeration or
  //   pointer to member type and VQ is either volatile or
  //   empty, there exist candidate operator functions of the form
  //
  //        VQ T&      operator=(VQ T&, T);
  void addAssignmentMemberPointerOrEnumeralOverloads() {
    /// Set of (canonical) types that we've already handled.
    llvm::SmallPtrSet<QualType, 8> AddedTypes;

    for (unsigned ArgIdx = 0; ArgIdx < 2; ++ArgIdx) {
      for (QualType EnumTy : CandidateTypes[ArgIdx].enumeration_types()) {
        if (!AddedTypes.insert(S.Context.getCanonicalType(EnumTy)).second)
          continue;

        AddBuiltinAssignmentOperatorCandidates(S, EnumTy, Args, CandidateSet);
      }

      for (QualType MemPtrTy : CandidateTypes[ArgIdx].member_pointer_types()) {
        if (!AddedTypes.insert(S.Context.getCanonicalType(MemPtrTy)).second)
          continue;

        AddBuiltinAssignmentOperatorCandidates(S, MemPtrTy, Args, CandidateSet);
      }
    }
  }

  // C++ [over.built]p19:
  //
  //   For every pair (T, VQ), where T is any type and VQ is either
  //   volatile or empty, there exist candidate operator functions
  //   of the form
  //
  //        T*VQ&      operator=(T*VQ&, T*);
  //
  // C++ [over.built]p21:
  //
  //   For every pair (T, VQ), where T is a cv-qualified or
  //   cv-unqualified object type and VQ is either volatile or
  //   empty, there exist candidate operator functions of the form
  //
  //        T*VQ&      operator+=(T*VQ&, ptrdiff_t);
  //        T*VQ&      operator-=(T*VQ&, ptrdiff_t);
  void addAssignmentPointerOverloads(bool isEqualOp) {
    /// Set of (canonical) types that we've already handled.
    llvm::SmallPtrSet<QualType, 8> AddedTypes;

    for (QualType PtrTy : CandidateTypes[0].pointer_types()) {
      // If this is operator=, keep track of the builtin candidates we added.
      if (isEqualOp)
        AddedTypes.insert(S.Context.getCanonicalType(PtrTy));
      else if (!PtrTy->getPointeeType()->isObjectType())
        continue;

      // non-volatile version
      QualType ParamTypes[2] = {
          S.Context.getLValueReferenceType(PtrTy),
          isEqualOp ? PtrTy : S.Context.getPointerDiffType(),
      };
      S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                            /*IsAssignmentOperator=*/ isEqualOp);

      bool NeedVolatile = !PtrTy.isVolatileQualified() &&
                          VisibleTypeConversionsQuals.hasVolatile();
      if (NeedVolatile) {
        // volatile version
        ParamTypes[0] =
            S.Context.getLValueReferenceType(S.Context.getVolatileType(PtrTy));
        S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                              /*IsAssignmentOperator=*/isEqualOp);
      }

      if (!PtrTy.isRestrictQualified() &&
          VisibleTypeConversionsQuals.hasRestrict()) {
        // restrict version
        ParamTypes[0] =
            S.Context.getLValueReferenceType(S.Context.getRestrictType(PtrTy));
        S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                              /*IsAssignmentOperator=*/isEqualOp);

        if (NeedVolatile) {
          // volatile restrict version
          ParamTypes[0] =
              S.Context.getLValueReferenceType(S.Context.getCVRQualifiedType(
                  PtrTy, (Qualifiers::Volatile | Qualifiers::Restrict)));
          S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                                /*IsAssignmentOperator=*/isEqualOp);
        }
      }
    }

    if (isEqualOp) {
      for (QualType PtrTy : CandidateTypes[1].pointer_types()) {
        // Make sure we don't add the same candidate twice.
        if (!AddedTypes.insert(S.Context.getCanonicalType(PtrTy)).second)
          continue;

        QualType ParamTypes[2] = {
            S.Context.getLValueReferenceType(PtrTy),
            PtrTy,
        };

        // non-volatile version
        S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                              /*IsAssignmentOperator=*/true);

        bool NeedVolatile = !PtrTy.isVolatileQualified() &&
                            VisibleTypeConversionsQuals.hasVolatile();
        if (NeedVolatile) {
          // volatile version
          ParamTypes[0] = S.Context.getLValueReferenceType(
              S.Context.getVolatileType(PtrTy));
          S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                                /*IsAssignmentOperator=*/true);
        }

        if (!PtrTy.isRestrictQualified() &&
            VisibleTypeConversionsQuals.hasRestrict()) {
          // restrict version
          ParamTypes[0] = S.Context.getLValueReferenceType(
              S.Context.getRestrictType(PtrTy));
          S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                                /*IsAssignmentOperator=*/true);

          if (NeedVolatile) {
            // volatile restrict version
            ParamTypes[0] =
                S.Context.getLValueReferenceType(S.Context.getCVRQualifiedType(
                    PtrTy, (Qualifiers::Volatile | Qualifiers::Restrict)));
            S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                                  /*IsAssignmentOperator=*/true);
          }
        }
      }
    }
  }

  // C++ [over.built]p18:
  //
  //   For every triple (L, VQ, R), where L is an arithmetic type,
  //   VQ is either volatile or empty, and R is a promoted
  //   arithmetic type, there exist candidate operator functions of
  //   the form
  //
  //        VQ L&      operator=(VQ L&, R);
  //        VQ L&      operator*=(VQ L&, R);
  //        VQ L&      operator/=(VQ L&, R);
  //        VQ L&      operator+=(VQ L&, R);
  //        VQ L&      operator-=(VQ L&, R);
  void addAssignmentArithmeticOverloads(bool isEqualOp) {
    if (!HasArithmeticOrEnumeralCandidateType)
      return;

    for (unsigned Left = 0; Left < NumArithmeticTypes; ++Left) {
      for (unsigned Right = FirstPromotedArithmeticType;
           Right < LastPromotedArithmeticType; ++Right) {
        QualType ParamTypes[2];
        ParamTypes[1] = ArithmeticTypes[Right];
        auto LeftBaseTy = AdjustAddressSpaceForBuiltinOperandType(
            S, ArithmeticTypes[Left], Args[0]);

        forAllQualifierCombinations(
            VisibleTypeConversionsQuals, [&](QualifiersAndAtomic Quals) {
              ParamTypes[0] =
                  makeQualifiedLValueReferenceType(LeftBaseTy, Quals, S);
              S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                                    /*IsAssignmentOperator=*/isEqualOp);
            });
      }
    }

    // Extension: Add the binary operators =, +=, -=, *=, /= for vector types.
    for (QualType Vec1Ty : CandidateTypes[0].vector_types())
      for (QualType Vec2Ty : CandidateTypes[0].vector_types()) {
        QualType ParamTypes[2];
        ParamTypes[1] = Vec2Ty;
        // Add this built-in operator as a candidate (VQ is empty).
        ParamTypes[0] = S.Context.getLValueReferenceType(Vec1Ty);
        S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                              /*IsAssignmentOperator=*/isEqualOp);

        // Add this built-in operator as a candidate (VQ is 'volatile').
        if (VisibleTypeConversionsQuals.hasVolatile()) {
          ParamTypes[0] = S.Context.getVolatileType(Vec1Ty);
          ParamTypes[0] = S.Context.getLValueReferenceType(ParamTypes[0]);
          S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                                /*IsAssignmentOperator=*/isEqualOp);
        }
      }
  }

  // C++ [over.built]p22:
  //
  //   For every triple (L, VQ, R), where L is an integral type, VQ
  //   is either volatile or empty, and R is a promoted integral
  //   type, there exist candidate operator functions of the form
  //
  //        VQ L&       operator%=(VQ L&, R);
  //        VQ L&       operator<<=(VQ L&, R);
  //        VQ L&       operator>>=(VQ L&, R);
  //        VQ L&       operator&=(VQ L&, R);
  //        VQ L&       operator^=(VQ L&, R);
  //        VQ L&       operator|=(VQ L&, R);
  void addAssignmentIntegralOverloads() {
    if (!HasArithmeticOrEnumeralCandidateType)
      return;

    for (unsigned Left = FirstIntegralType; Left < LastIntegralType; ++Left) {
      for (unsigned Right = FirstPromotedIntegralType;
           Right < LastPromotedIntegralType; ++Right) {
        QualType ParamTypes[2];
        ParamTypes[1] = ArithmeticTypes[Right];
        auto LeftBaseTy = AdjustAddressSpaceForBuiltinOperandType(
            S, ArithmeticTypes[Left], Args[0]);

        forAllQualifierCombinations(
            VisibleTypeConversionsQuals, [&](QualifiersAndAtomic Quals) {
              ParamTypes[0] =
                  makeQualifiedLValueReferenceType(LeftBaseTy, Quals, S);
              S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
            });
      }
    }
  }

  // C++ [over.operator]p23:
  //
  //   There also exist candidate operator functions of the form
  //
  //        bool        operator!(bool);
  //        bool        operator&&(bool, bool);
  //        bool        operator||(bool, bool);
  void addExclaimOverload() {
    QualType ParamTy = S.Context.BoolTy;
    S.AddBuiltinCandidate(&ParamTy, Args, CandidateSet,
                          /*IsAssignmentOperator=*/false,
                          /*NumContextualBoolArguments=*/1);
  }
  void addAmpAmpOrPipePipeOverload() {
    QualType ParamTypes[2] = { S.Context.BoolTy, S.Context.BoolTy };
    S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet,
                          /*IsAssignmentOperator=*/false,
                          /*NumContextualBoolArguments=*/2);
  }

  // C++ [over.built]p13:
  //
  //   For every cv-qualified or cv-unqualified object type T there
  //   exist candidate operator functions of the form
  //
  //        T*         operator+(T*, ptrdiff_t);     [ABOVE]
  //        T&         operator[](T*, ptrdiff_t);
  //        T*         operator-(T*, ptrdiff_t);     [ABOVE]
  //        T*         operator+(ptrdiff_t, T*);     [ABOVE]
  //        T&         operator[](ptrdiff_t, T*);
  void addSubscriptOverloads() {
    for (QualType PtrTy : CandidateTypes[0].pointer_types()) {
      QualType ParamTypes[2] = {PtrTy, S.Context.getPointerDiffType()};
      QualType PointeeType = PtrTy->getPointeeType();
      if (!PointeeType->isObjectType())
        continue;

      // T& operator[](T*, ptrdiff_t)
      S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
    }

    for (QualType PtrTy : CandidateTypes[1].pointer_types()) {
      QualType ParamTypes[2] = {S.Context.getPointerDiffType(), PtrTy};
      QualType PointeeType = PtrTy->getPointeeType();
      if (!PointeeType->isObjectType())
        continue;

      // T& operator[](ptrdiff_t, T*)
      S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
    }
  }

  // C++ [over.built]p11:
  //    For every quintuple (C1, C2, T, CV1, CV2), where C2 is a class type,
  //    C1 is the same type as C2 or is a derived class of C2, T is an object
  //    type or a function type, and CV1 and CV2 are cv-qualifier-seqs,
  //    there exist candidate operator functions of the form
  //
  //      CV12 T& operator->*(CV1 C1*, CV2 T C2::*);
  //
  //    where CV12 is the union of CV1 and CV2.
  void addArrowStarOverloads() {
    for (QualType PtrTy : CandidateTypes[0].pointer_types()) {
      QualType C1Ty = PtrTy;
      QualType C1;
      QualifierCollector Q1;
      C1 = QualType(Q1.strip(C1Ty->getPointeeType()), 0);
      if (!isa<RecordType>(C1))
        continue;
      // heuristic to reduce number of builtin candidates in the set.
      // Add volatile/restrict version only if there are conversions to a
      // volatile/restrict type.
      if (!VisibleTypeConversionsQuals.hasVolatile() && Q1.hasVolatile())
        continue;
      if (!VisibleTypeConversionsQuals.hasRestrict() && Q1.hasRestrict())
        continue;
      for (QualType MemPtrTy : CandidateTypes[1].member_pointer_types()) {
        const MemberPointerType *mptr = cast<MemberPointerType>(MemPtrTy);
        QualType C2 = QualType(mptr->getClass(), 0);
        C2 = C2.getUnqualifiedType();
        if (C1 != C2 && !S.IsDerivedFrom(CandidateSet.getLocation(), C1, C2))
          break;
        QualType ParamTypes[2] = {PtrTy, MemPtrTy};
        // build CV12 T&
        QualType T = mptr->getPointeeType();
        if (!VisibleTypeConversionsQuals.hasVolatile() &&
            T.isVolatileQualified())
          continue;
        if (!VisibleTypeConversionsQuals.hasRestrict() &&
            T.isRestrictQualified())
          continue;
        T = Q1.apply(S.Context, T);
        S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
      }
    }
  }

  // Note that we don't consider the first argument, since it has been
  // contextually converted to bool long ago. The candidates below are
  // therefore added as binary.
  //
  // C++ [over.built]p25:
  //   For every type T, where T is a pointer, pointer-to-member, or scoped
  //   enumeration type, there exist candidate operator functions of the form
  //
  //        T        operator?(bool, T, T);
  //
  void addConditionalOperatorOverloads() {
    /// Set of (canonical) types that we've already handled.
    llvm::SmallPtrSet<QualType, 8> AddedTypes;

    for (unsigned ArgIdx = 0; ArgIdx < 2; ++ArgIdx) {
      for (QualType PtrTy : CandidateTypes[ArgIdx].pointer_types()) {
        if (!AddedTypes.insert(S.Context.getCanonicalType(PtrTy)).second)
          continue;

        QualType ParamTypes[2] = {PtrTy, PtrTy};
        S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
      }

      for (QualType MemPtrTy : CandidateTypes[ArgIdx].member_pointer_types()) {
        if (!AddedTypes.insert(S.Context.getCanonicalType(MemPtrTy)).second)
          continue;

        QualType ParamTypes[2] = {MemPtrTy, MemPtrTy};
        S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
      }

      if (S.getLangOpts().CPlusPlus11) {
        for (QualType EnumTy : CandidateTypes[ArgIdx].enumeration_types()) {
          if (!EnumTy->castAs<EnumType>()->getDecl()->isScoped())
            continue;

          if (!AddedTypes.insert(S.Context.getCanonicalType(EnumTy)).second)
            continue;

          QualType ParamTypes[2] = {EnumTy, EnumTy};
          S.AddBuiltinCandidate(ParamTypes, Args, CandidateSet);
        }
      }
    }
  }
};

} // end anonymous namespace

/// AddBuiltinOperatorCandidates - Add the appropriate built-in
/// operator overloads to the candidate set (C++ [over.built]), based
/// on the operator @p Op and the arguments given. For example, if the
/// operator is a binary '+', this routine might add "int
/// operator+(int, int)" to cover integer addition.
void Sema::AddBuiltinOperatorCandidates(OverloadedOperatorKind Op,
                                        SourceLocation OpLoc,
                                        ArrayRef<Expr *> Args,
                                        OverloadCandidateSet &CandidateSet) {
  // Find all of the types that the arguments can convert to, but only
  // if the operator we're looking at has built-in operator candidates
  // that make use of these types. Also record whether we encounter non-record
  // candidate types or either arithmetic or enumeral candidate types.
  QualifiersAndAtomic VisibleTypeConversionsQuals;
  VisibleTypeConversionsQuals.addConst();
  for (unsigned ArgIdx = 0, N = Args.size(); ArgIdx != N; ++ArgIdx) {
    VisibleTypeConversionsQuals += CollectVRQualifiers(Context, Args[ArgIdx]);
    if (Args[ArgIdx]->getType()->isAtomicType())
      VisibleTypeConversionsQuals.addAtomic();
  }

  bool HasNonRecordCandidateType = false;
  bool HasArithmeticOrEnumeralCandidateType = false;
  SmallVector<BuiltinCandidateTypeSet, 2> CandidateTypes;
  for (unsigned ArgIdx = 0, N = Args.size(); ArgIdx != N; ++ArgIdx) {
    CandidateTypes.emplace_back(*this);
    CandidateTypes[ArgIdx].AddTypesConvertedFrom(Args[ArgIdx]->getType(),
                                                 OpLoc,
                                                 true,
                                                 (Op == OO_Exclaim ||
                                                  Op == OO_AmpAmp ||
                                                  Op == OO_PipePipe),
                                                 VisibleTypeConversionsQuals);
    HasNonRecordCandidateType = HasNonRecordCandidateType ||
        CandidateTypes[ArgIdx].hasNonRecordTypes();
    HasArithmeticOrEnumeralCandidateType =
        HasArithmeticOrEnumeralCandidateType ||
        CandidateTypes[ArgIdx].hasArithmeticOrEnumeralTypes();
  }

  // Exit early when no non-record types have been added to the candidate set
  // for any of the arguments to the operator.
  //
  // We can't exit early for !, ||, or &&, since there we have always have
  // 'bool' overloads.
  if (!HasNonRecordCandidateType &&
      !(Op == OO_Exclaim || Op == OO_AmpAmp || Op == OO_PipePipe))
    return;

  // Setup an object to manage the common state for building overloads.
  BuiltinOperatorOverloadBuilder OpBuilder(*this, Args,
                                           VisibleTypeConversionsQuals,
                                           HasArithmeticOrEnumeralCandidateType,
                                           CandidateTypes, CandidateSet);

  // Dispatch over the operation to add in only those overloads which apply.
  switch (Op) {
  case OO_None:
  case NUM_OVERLOADED_OPERATORS:
    llvm_unreachable("Expected an overloaded operator");

  case OO_New:
  case OO_Delete:
  case OO_Array_New:
  case OO_Array_Delete:
  case OO_Call:
    llvm_unreachable(
                    "Special operators don't use AddBuiltinOperatorCandidates");

  case OO_Comma:
  case OO_Arrow:
  case OO_Coawait:
    // C++ [over.match.oper]p3:
    //   -- For the operator ',', the unary operator '&', the
    //      operator '->', or the operator 'co_await', the
    //      built-in candidates set is empty.
    break;

  case OO_Plus: // '+' is either unary or binary
    if (Args.size() == 1)
      OpBuilder.addUnaryPlusPointerOverloads();
    [[fallthrough]];

  case OO_Minus: // '-' is either unary or binary
    if (Args.size() == 1) {
      OpBuilder.addUnaryPlusOrMinusArithmeticOverloads();
    } else {
      OpBuilder.addBinaryPlusOrMinusPointerOverloads(Op);
      OpBuilder.addGenericBinaryArithmeticOverloads();
      OpBuilder.addMatrixBinaryArithmeticOverloads();
    }
    break;

  case OO_Star: // '*' is either unary or binary
    if (Args.size() == 1)
      OpBuilder.addUnaryStarPointerOverloads();
    else {
      OpBuilder.addGenericBinaryArithmeticOverloads();
      OpBuilder.addMatrixBinaryArithmeticOverloads();
    }
    break;

  case OO_Slash:
    OpBuilder.addGenericBinaryArithmeticOverloads();
    break;

  case OO_PlusPlus:
  case OO_MinusMinus:
    OpBuilder.addPlusPlusMinusMinusArithmeticOverloads(Op);
    OpBuilder.addPlusPlusMinusMinusPointerOverloads();
    break;

  case OO_EqualEqual:
  case OO_ExclaimEqual:
    OpBuilder.addEqualEqualOrNotEqualMemberPointerOrNullptrOverloads();
    OpBuilder.addGenericBinaryPointerOrEnumeralOverloads(/*IsSpaceship=*/false);
    OpBuilder.addGenericBinaryArithmeticOverloads();
    break;

  case OO_Less:
  case OO_Greater:
  case OO_LessEqual:
  case OO_GreaterEqual:
    OpBuilder.addGenericBinaryPointerOrEnumeralOverloads(/*IsSpaceship=*/false);
    OpBuilder.addGenericBinaryArithmeticOverloads();
    break;

  case OO_Spaceship:
    OpBuilder.addGenericBinaryPointerOrEnumeralOverloads(/*IsSpaceship=*/true);
    OpBuilder.addThreeWayArithmeticOverloads();
    break;

  case OO_Percent:
  case OO_Caret:
  case OO_Pipe:
  case OO_LessLess:
  case OO_GreaterGreater:
    OpBuilder.addBinaryBitwiseArithmeticOverloads();
    break;

  case OO_Amp: // '&' is either unary or binary
    if (Args.size() == 1)
      // C++ [over.match.oper]p3:
      //   -- For the operator ',', the unary operator '&', or the
      //      operator '->', the built-in candidates set is empty.
      break;

    OpBuilder.addBinaryBitwiseArithmeticOverloads();
    break;

  case OO_Tilde:
    OpBuilder.addUnaryTildePromotedIntegralOverloads();
    break;

  case OO_Equal:
    OpBuilder.addAssignmentMemberPointerOrEnumeralOverloads();
    [[fallthrough]];

  case OO_PlusEqual:
  case OO_MinusEqual:
    OpBuilder.addAssignmentPointerOverloads(Op == OO_Equal);
    [[fallthrough]];

  case OO_StarEqual:
  case OO_SlashEqual:
    OpBuilder.addAssignmentArithmeticOverloads(Op == OO_Equal);
    break;

  case OO_PercentEqual:
  case OO_LessLessEqual:
  case OO_GreaterGreaterEqual:
  case OO_AmpEqual:
  case OO_CaretEqual:
  case OO_PipeEqual:
    OpBuilder.addAssignmentIntegralOverloads();
    break;

  case OO_Exclaim:
    OpBuilder.addExclaimOverload();
    break;

  case OO_AmpAmp:
  case OO_PipePipe:
    OpBuilder.addAmpAmpOrPipePipeOverload();
    break;

  case OO_Subscript:
    if (Args.size() == 2)
      OpBuilder.addSubscriptOverloads();
    break;

  case OO_ArrowStar:
    OpBuilder.addArrowStarOverloads();
    break;

  case OO_Conditional:
    OpBuilder.addConditionalOperatorOverloads();
    OpBuilder.addGenericBinaryArithmeticOverloads();
    break;
  }
}

/// Add function candidates found via argument-dependent lookup
/// to the set of overloading candidates.
///
/// This routine performs argument-dependent name lookup based on the
/// given function name (which may also be an operator name) and adds
/// all of the overload candidates found by ADL to the overload
/// candidate set (C++ [basic.lookup.argdep]).
void
Sema::AddArgumentDependentLookupCandidates(DeclarationName Name,
                                           SourceLocation Loc,
                                           ArrayRef<Expr *> Args,
                                 TemplateArgumentListInfo *ExplicitTemplateArgs,
                                           OverloadCandidateSet& CandidateSet,
                                           bool PartialOverloading) {
  ADLResult Fns;

  // FIXME: This approach for uniquing ADL results (and removing
  // redundant candidates from the set) relies on pointer-equality,
  // which means we need to key off the canonical decl.  However,
  // always going back to the canonical decl might not get us the
  // right set of default arguments.  What default arguments are
  // we supposed to consider on ADL candidates, anyway?

  // FIXME: Pass in the explicit template arguments?
  ArgumentDependentLookup(Name, Loc, Args, Fns);

  // Erase all of the candidates we already knew about.
  for (OverloadCandidateSet::iterator Cand = CandidateSet.begin(),
                                   CandEnd = CandidateSet.end();
       Cand != CandEnd; ++Cand)
    if (Cand->Function) {
      Fns.erase(Cand->Function);
      if (FunctionTemplateDecl *FunTmpl = Cand->Function->getPrimaryTemplate())
        Fns.erase(FunTmpl);
    }

  // For each of the ADL candidates we found, add it to the overload
  // set.
  for (ADLResult::iterator I = Fns.begin(), E = Fns.end(); I != E; ++I) {
    DeclAccessPair FoundDecl = DeclAccessPair::make(*I, AS_none);

    if (FunctionDecl *FD = dyn_cast<FunctionDecl>(*I)) {
      if (ExplicitTemplateArgs)
        continue;

      AddOverloadCandidate(
          FD, FoundDecl, Args, CandidateSet, /*SuppressUserConversions=*/false,
          PartialOverloading, /*AllowExplicit=*/true,
          /*AllowExplicitConversion=*/false, ADLCallKind::UsesADL);
      if (CandidateSet.getRewriteInfo().shouldAddReversed(*this, Args, FD)) {
        AddOverloadCandidate(
            FD, FoundDecl, {Args[1], Args[0]}, CandidateSet,
            /*SuppressUserConversions=*/false, PartialOverloading,
            /*AllowExplicit=*/true, /*AllowExplicitConversion=*/false,
            ADLCallKind::UsesADL, std::nullopt,
            OverloadCandidateParamOrder::Reversed);
      }
    } else {
      auto *FTD = cast<FunctionTemplateDecl>(*I);
      AddTemplateOverloadCandidate(
          FTD, FoundDecl, ExplicitTemplateArgs, Args, CandidateSet,
          /*SuppressUserConversions=*/false, PartialOverloading,
          /*AllowExplicit=*/true, ADLCallKind::UsesADL);
      if (CandidateSet.getRewriteInfo().shouldAddReversed(
              *this, Args, FTD->getTemplatedDecl())) {
        AddTemplateOverloadCandidate(
            FTD, FoundDecl, ExplicitTemplateArgs, {Args[1], Args[0]},
            CandidateSet, /*SuppressUserConversions=*/false, PartialOverloading,
            /*AllowExplicit=*/true, ADLCallKind::UsesADL,
            OverloadCandidateParamOrder::Reversed);
      }
    }
  }
}

namespace {
enum class Comparison { Equal, Better, Worse };
}

/// Compares the enable_if attributes of two FunctionDecls, for the purposes of
/// overload resolution.
///
/// Cand1's set of enable_if attributes are said to be "better" than Cand2's iff
/// Cand1's first N enable_if attributes have precisely the same conditions as
/// Cand2's first N enable_if attributes (where N = the number of enable_if
/// attributes on Cand2), and Cand1 has more than N enable_if attributes.
///
/// Note that you can have a pair of candidates such that Cand1's enable_if
/// attributes are worse than Cand2's, and Cand2's enable_if attributes are
/// worse than Cand1's.
static Comparison compareEnableIfAttrs(const Sema &S, const FunctionDecl *Cand1,
                                       const FunctionDecl *Cand2) {
  // Common case: One (or both) decls don't have enable_if attrs.
  bool Cand1Attr = Cand1->hasAttr<EnableIfAttr>();
  bool Cand2Attr = Cand2->hasAttr<EnableIfAttr>();
  if (!Cand1Attr || !Cand2Attr) {
    if (Cand1Attr == Cand2Attr)
      return Comparison::Equal;
    return Cand1Attr ? Comparison::Better : Comparison::Worse;
  }

  auto Cand1Attrs = Cand1->specific_attrs<EnableIfAttr>();
  auto Cand2Attrs = Cand2->specific_attrs<EnableIfAttr>();

  llvm::FoldingSetNodeID Cand1ID, Cand2ID;
  for (auto Pair : zip_longest(Cand1Attrs, Cand2Attrs)) {
    std::optional<EnableIfAttr *> Cand1A = std::get<0>(Pair);
    std::optional<EnableIfAttr *> Cand2A = std::get<1>(Pair);

    // It's impossible for Cand1 to be better than (or equal to) Cand2 if Cand1
    // has fewer enable_if attributes than Cand2, and vice versa.
    if (!Cand1A)
      return Comparison::Worse;
    if (!Cand2A)
      return Comparison::Better;

    Cand1ID.clear();
    Cand2ID.clear();

    (*Cand1A)->getCond()->Profile(Cand1ID, S.getASTContext(), true);
    (*Cand2A)->getCond()->Profile(Cand2ID, S.getASTContext(), true);
    if (Cand1ID != Cand2ID)
      return Comparison::Worse;
  }

  return Comparison::Equal;
}

static Comparison
isBetterMultiversionCandidate(const OverloadCandidate &Cand1,
                              const OverloadCandidate &Cand2) {
  if (!Cand1.Function || !Cand1.Function->isMultiVersion() || !Cand2.Function ||
      !Cand2.Function->isMultiVersion())
    return Comparison::Equal;

  // If both are invalid, they are equal. If one of them is invalid, the other
  // is better.
  if (Cand1.Function->isInvalidDecl()) {
    if (Cand2.Function->isInvalidDecl())
      return Comparison::Equal;
    return Comparison::Worse;
  }
  if (Cand2.Function->isInvalidDecl())
    return Comparison::Better;

  // If this is a cpu_dispatch/cpu_specific multiversion situation, prefer
  // cpu_dispatch, else arbitrarily based on the identifiers.
  bool Cand1CPUDisp = Cand1.Function->hasAttr<CPUDispatchAttr>();
  bool Cand2CPUDisp = Cand2.Function->hasAttr<CPUDispatchAttr>();
  const auto *Cand1CPUSpec = Cand1.Function->getAttr<CPUSpecificAttr>();
  const auto *Cand2CPUSpec = Cand2.Function->getAttr<CPUSpecificAttr>();

  if (!Cand1CPUDisp && !Cand2CPUDisp && !Cand1CPUSpec && !Cand2CPUSpec)
    return Comparison::Equal;

  if (Cand1CPUDisp && !Cand2CPUDisp)
    return Comparison::Better;
  if (Cand2CPUDisp && !Cand1CPUDisp)
    return Comparison::Worse;

  if (Cand1CPUSpec && Cand2CPUSpec) {
    if (Cand1CPUSpec->cpus_size() != Cand2CPUSpec->cpus_size())
      return Cand1CPUSpec->cpus_size() < Cand2CPUSpec->cpus_size()
                 ? Comparison::Better
                 : Comparison::Worse;

    std::pair<CPUSpecificAttr::cpus_iterator, CPUSpecificAttr::cpus_iterator>
        FirstDiff = std::mismatch(
            Cand1CPUSpec->cpus_begin(), Cand1CPUSpec->cpus_end(),
            Cand2CPUSpec->cpus_begin(),
            [](const IdentifierInfo *LHS, const IdentifierInfo *RHS) {
              return LHS->getName() == RHS->getName();
            });

    assert(FirstDiff.first != Cand1CPUSpec->cpus_end() &&
           "Two different cpu-specific versions should not have the same "
           "identifier list, otherwise they'd be the same decl!");
    return (*FirstDiff.first)->getName() < (*FirstDiff.second)->getName()
               ? Comparison::Better
               : Comparison::Worse;
  }
  llvm_unreachable("No way to get here unless both had cpu_dispatch");
}

/// Compute the type of the implicit object parameter for the given function,
/// if any. Returns std::nullopt if there is no implicit object parameter, and a
/// null QualType if there is a 'matches anything' implicit object parameter.
static std::optional<QualType>
getImplicitObjectParamType(ASTContext &Context, const FunctionDecl *F) {
  if (!isa<CXXMethodDecl>(F) || isa<CXXConstructorDecl>(F))
    return std::nullopt;

  auto *M = cast<CXXMethodDecl>(F);
  // Static member functions' object parameters match all types.
  if (M->isStatic())
    return QualType();

  QualType T = M->getThisObjectType();
  if (M->getRefQualifier() == RQ_RValue)
    return Context.getRValueReferenceType(T);
  return Context.getLValueReferenceType(T);
}

static bool haveSameParameterTypes(ASTContext &Context, const FunctionDecl *F1,
                                   const FunctionDecl *F2, unsigned NumParams) {
  if (declaresSameEntity(F1, F2))
    return true;

  auto NextParam = [&](const FunctionDecl *F, unsigned &I, bool First) {
    if (First) {
      if (std::optional<QualType> T = getImplicitObjectParamType(Context, F))
        return *T;
    }
    assert(I < F->getNumParams());
    return F->getParamDecl(I++)->getType();
  };

  unsigned I1 = 0, I2 = 0;
  for (unsigned I = 0; I != NumParams; ++I) {
    QualType T1 = NextParam(F1, I1, I == 0);
    QualType T2 = NextParam(F2, I2, I == 0);
    assert(!T1.isNull() && !T2.isNull() && "Unexpected null param types");
    if (!Context.hasSameUnqualifiedType(T1, T2))
      return false;
  }
  return true;
}

/// We're allowed to use constraints partial ordering only if the candidates
/// have the same parameter types:
/// [over.match.best]p2.6
/// F1 and F2 are non-template functions with the same parameter-type-lists,
/// and F1 is more constrained than F2 [...]
static bool sameFunctionParameterTypeLists(Sema &S,
                                          const OverloadCandidate &Cand1,
                                          const OverloadCandidate &Cand2) {
  if (Cand1.Function && Cand2.Function) {
    auto *PT1 = cast<FunctionProtoType>(Cand1.Function->getFunctionType());
    auto *PT2 = cast<FunctionProtoType>(Cand2.Function->getFunctionType());
    if (PT1->getNumParams() == PT2->getNumParams() &&
        PT1->isVariadic() == PT2->isVariadic() &&
        S.FunctionParamTypesAreEqual(PT1, PT2, nullptr,
                                     Cand1.isReversed() ^ Cand2.isReversed()))
      return true;
  }
  return false;
}

/// isBetterOverloadCandidate - Determines whether the first overload
/// candidate is a better candidate than the second (C++ 13.3.3p1).
bool clang::isBetterOverloadCandidate(
    Sema &S, const OverloadCandidate &Cand1, const OverloadCandidate &Cand2,
    SourceLocation Loc, OverloadCandidateSet::CandidateSetKind Kind) {
  // Define viable functions to be better candidates than non-viable
  // functions.
  if (!Cand2.Viable)
    return Cand1.Viable;
  else if (!Cand1.Viable)
    return false;

  // [CUDA] A function with 'never' preference is marked not viable, therefore
  // is never shown up here. The worst preference shown up here is 'wrong side',
  // e.g. an H function called by a HD function in device compilation. This is
  // valid AST as long as the HD function is not emitted, e.g. it is an inline
  // function which is called only by an H function. A deferred diagnostic will
  // be triggered if it is emitted. However a wrong-sided function is still
  // a viable candidate here.
  //
  // If Cand1 can be emitted and Cand2 cannot be emitted in the current
  // context, Cand1 is better than Cand2. If Cand1 can not be emitted and Cand2
  // can be emitted, Cand1 is not better than Cand2. This rule should have
  // precedence over other rules.
  //
  // If both Cand1 and Cand2 can be emitted, or neither can be emitted, then
  // other rules should be used to determine which is better. This is because
  // host/device based overloading resolution is mostly for determining
  // viability of a function. If two functions are both viable, other factors
  // should take precedence in preference, e.g. the standard-defined preferences
  // like argument conversion ranks or enable_if partial-ordering. The
  // preference for pass-object-size parameters is probably most similar to a
  // type-based-overloading decision and so should take priority.
  //
  // If other rules cannot determine which is better, CUDA preference will be
  // used again to determine which is better.
  //
  // TODO: Currently IdentifyCUDAPreference does not return correct values
  // for functions called in global variable initializers due to missing
  // correct context about device/host. Therefore we can only enforce this
  // rule when there is a caller. We should enforce this rule for functions
  // in global variable initializers once proper context is added.
  //
  // TODO: We can only enable the hostness based overloading resolution when
  // -fgpu-exclude-wrong-side-overloads is on since this requires deferring
  // overloading resolution diagnostics.
  if (S.getLangOpts().CUDA && Cand1.Function && Cand2.Function &&
      S.getLangOpts().GPUExcludeWrongSideOverloads) {
    if (FunctionDecl *Caller = S.getCurFunctionDecl(/*AllowLambda=*/true)) {
      bool IsCallerImplicitHD = Sema::isCUDAImplicitHostDeviceFunction(Caller);
      bool IsCand1ImplicitHD =
          Sema::isCUDAImplicitHostDeviceFunction(Cand1.Function);
      bool IsCand2ImplicitHD =
          Sema::isCUDAImplicitHostDeviceFunction(Cand2.Function);
      auto P1 = S.IdentifyCUDAPreference(Caller, Cand1.Function);
      auto P2 = S.IdentifyCUDAPreference(Caller, Cand2.Function);
      assert(P1 != Sema::CFP_Never && P2 != Sema::CFP_Never);
      // The implicit HD function may be a function in a system header which
      // is forced by pragma. In device compilation, if we prefer HD candidates
      // over wrong-sided candidates, overloading resolution may change, which
      // may result in non-deferrable diagnostics. As a workaround, we let
      // implicit HD candidates take equal preference as wrong-sided candidates.
      // This will preserve the overloading resolution.
      // TODO: We still need special handling of implicit HD functions since
      // they may incur other diagnostics to be deferred. We should make all
      // host/device related diagnostics deferrable and remove special handling
      // of implicit HD functions.
      auto EmitThreshold =
          (S.getLangOpts().CUDAIsDevice && IsCallerImplicitHD &&
           (IsCand1ImplicitHD || IsCand2ImplicitHD))
              ? Sema::CFP_Never
              : Sema::CFP_WrongSide;
      auto Cand1Emittable = P1 > EmitThreshold;
      auto Cand2Emittable = P2 > EmitThreshold;
      if (Cand1Emittable && !Cand2Emittable)
        return true;
      if (!Cand1Emittable && Cand2Emittable)
        return false;
    }
  }

  // C++ [over.match.best]p1: (Changed in C++23)
  //
  //   -- if F is a static member function, ICS1(F) is defined such
  //      that ICS1(F) is neither better nor worse than ICS1(G) for
  //      any function G, and, symmetrically, ICS1(G) is neither
  //      better nor worse than ICS1(F).
  unsigned StartArg = 0;
  if (Cand1.IgnoreObjectArgument || Cand2.IgnoreObjectArgument)
    StartArg = 1;

  auto IsIllFormedConversion = [&](const ImplicitConversionSequence &ICS) {
    // We don't allow incompatible pointer conversions in C++.
    if (!S.getLangOpts().CPlusPlus)
      return ICS.isStandard() &&
             ICS.Standard.Second == ICK_Incompatible_Pointer_Conversion;

    // The only ill-formed conversion we allow in C++ is the string literal to
    // char* conversion, which is only considered ill-formed after C++11.
    return S.getLangOpts().CPlusPlus11 && !S.getLangOpts().WritableStrings &&
           hasDeprecatedStringLiteralToCharPtrConversion(ICS);
  };

  // Define functions that don't require ill-formed conversions for a given
  // argument to be better candidates than functions that do.
  unsigned NumArgs = Cand1.Conversions.size();
  assert(Cand2.Conversions.size() == NumArgs && "Overload candidate mismatch");
  bool HasBetterConversion = false;
  for (unsigned ArgIdx = StartArg; ArgIdx < NumArgs; ++ArgIdx) {
    bool Cand1Bad = IsIllFormedConversion(Cand1.Conversions[ArgIdx]);
    bool Cand2Bad = IsIllFormedConversion(Cand2.Conversions[ArgIdx]);
    if (Cand1Bad != Cand2Bad) {
      if (Cand1Bad)
        return false;
      HasBetterConversion = true;
    }
  }

  if (HasBetterConversion)
    return true;

  // C++ [over.match.best]p1:
  //   A viable function F1 is defined to be a better function than another
  //   viable function F2 if for all arguments i, ICSi(F1) is not a worse
  //   conversion sequence than ICSi(F2), and then...
  bool HasWorseConversion = false;
  for (unsigned ArgIdx = StartArg; ArgIdx < NumArgs; ++ArgIdx) {
    switch (CompareImplicitConversionSequences(S, Loc,
                                               Cand1.Conversions[ArgIdx],
                                               Cand2.Conversions[ArgIdx])) {
    case ImplicitConversionSequence::Better:
      // Cand1 has a better conversion sequence.
      HasBetterConversion = true;
      break;

    case ImplicitConversionSequence::Worse:
      if (Cand1.Function && Cand2.Function &&
          Cand1.isReversed() != Cand2.isReversed() &&
          haveSameParameterTypes(S.Context, Cand1.Function, Cand2.Function,
                                 NumArgs)) {
        // Work around large-scale breakage caused by considering reversed
        // forms of operator== in C++20:
        //
        // When comparing a function against a reversed function with the same
        // parameter types, if we have a better conversion for one argument and
        // a worse conversion for the other, the implicit conversion sequences
        // are treated as being equally good.
        //
        // This prevents a comparison function from being considered ambiguous
        // with a reversed form that is written in the same way.
        //
        // We diagnose this as an extension from CreateOverloadedBinOp.
        HasWorseConversion = true;
        break;
      }

      // Cand1 can't be better than Cand2.
      return false;

    case ImplicitConversionSequence::Indistinguishable:
      // Do nothing.
      break;
    }
  }

  //    -- for some argument j, ICSj(F1) is a better conversion sequence than
  //       ICSj(F2), or, if not that,
  if (HasBetterConversion && !HasWorseConversion)
    return true;

  //   -- the context is an initialization by user-defined conversion
  //      (see 8.5, 13.3.1.5) and the standard conversion sequence
  //      from the return type of F1 to the destination type (i.e.,
  //      the type of the entity being initialized) is a better
  //      conversion sequence than the standard conversion sequence
  //      from the return type of F2 to the destination type.
  if (Kind == OverloadCandidateSet::CSK_InitByUserDefinedConversion &&
      Cand1.Function && Cand2.Function &&
      isa<CXXConversionDecl>(Cand1.Function) &&
      isa<CXXConversionDecl>(Cand2.Function)) {
    // First check whether we prefer one of the conversion functions over the
    // other. This only distinguishes the results in non-standard, extension
    // cases such as the conversion from a lambda closure type to a function
    // pointer or block.
    ImplicitConversionSequence::CompareKind Result =
        compareConversionFunctions(S, Cand1.Function, Cand2.Function);
    if (Result == ImplicitConversionSequence::Indistinguishable)
      Result = CompareStandardConversionSequences(S, Loc,
                                                  Cand1.FinalConversion,
                                                  Cand2.FinalConversion);

    if (Result != ImplicitConversionSequence::Indistinguishable)
      return Result == ImplicitConversionSequence::Better;

    // FIXME: Compare kind of reference binding if conversion functions
    // convert to a reference type used in direct reference binding, per
    // C++14 [over.match.best]p1 section 2 bullet 3.
  }

  // FIXME: Work around a defect in the C++17 guaranteed copy elision wording,
  // as combined with the resolution to CWG issue 243.
  //
  // When the context is initialization by constructor ([over.match.ctor] or
  // either phase of [over.match.list]), a constructor is preferred over
  // a conversion function.
  if (Kind == OverloadCandidateSet::CSK_InitByConstructor && NumArgs == 1 &&
      Cand1.Function && Cand2.Function &&
      isa<CXXConstructorDecl>(Cand1.Function) !=
          isa<CXXConstructorDecl>(Cand2.Function))
    return isa<CXXConstructorDecl>(Cand1.Function);

  //    -- F1 is a non-template function and F2 is a function template
  //       specialization, or, if not that,
  bool Cand1IsSpecialization = Cand1.Function &&
                               Cand1.Function->getPrimaryTemplate();
  bool Cand2IsSpecialization = Cand2.Function &&
                               Cand2.Function->getPrimaryTemplate();
  if (Cand1IsSpecialization != Cand2IsSpecialization)
    return Cand2IsSpecialization;

  //   -- F1 and F2 are function template specializations, and the function
  //      template for F1 is more specialized than the template for F2
  //      according to the partial ordering rules described in 14.5.5.2, or,
  //      if not that,
  if (Cand1IsSpecialization && Cand2IsSpecialization) {
    if (FunctionTemplateDecl *BetterTemplate = S.getMoreSpecializedTemplate(
            Cand1.Function->getPrimaryTemplate(),
            Cand2.Function->getPrimaryTemplate(), Loc,
            isa<CXXConversionDecl>(Cand1.Function) ? TPOC_Conversion
                                                   : TPOC_Call,
            Cand1.ExplicitCallArguments, Cand2.ExplicitCallArguments,
            Cand1.isReversed() ^ Cand2.isReversed()))
      return BetterTemplate == Cand1.Function->getPrimaryTemplate();
  }

  //   -— F1 and F2 are non-template functions with the same
  //      parameter-type-lists, and F1 is more constrained than F2 [...],
  if (!Cand1IsSpecialization && !Cand2IsSpecialization &&
      sameFunctionParameterTypeLists(S, Cand1, Cand2)) {
    FunctionDecl *Function1 = Cand1.Function;
    FunctionDecl *Function2 = Cand2.Function;
    if (FunctionDecl *MF = Function1->getInstantiatedFromMemberFunction())
      Function1 = MF;
    if (FunctionDecl *MF = Function2->getInstantiatedFromMemberFunction())
      Function2 = MF;

    const Expr *RC1 = Function1->getTrailingRequiresClause();
    const Expr *RC2 = Function2->getTrailingRequiresClause();
    if (RC1 && RC2) {
      bool AtLeastAsConstrained1, AtLeastAsConstrained2;
      if (S.IsAtLeastAsConstrained(Function1, RC1, Function2, RC2,
                                   AtLeastAsConstrained1) ||
          S.IsAtLeastAsConstrained(Function2, RC2, Function1, RC1,
                                   AtLeastAsConstrained2))
        return false;
      if (AtLeastAsConstrained1 != AtLeastAsConstrained2)
        return AtLeastAsConstrained1;
    } else if (RC1 || RC2) {
      return RC1 != nullptr;
    }
  }

  //   -- F1 is a constructor for a class D, F2 is a constructor for a base
  //      class B of D, and for all arguments the corresponding parameters of
  //      F1 and F2 have the same type.
  // FIXME: Implement the "all parameters have the same type" check.
  bool Cand1IsInherited =
      isa_and_nonnull<ConstructorUsingShadowDecl>(Cand1.FoundDecl.getDecl());
  bool Cand2IsInherited =
      isa_and_nonnull<ConstructorUsingShadowDecl>(Cand2.FoundDecl.getDecl());
  if (Cand1IsInherited != Cand2IsInherited)
    return Cand2IsInherited;
  else if (Cand1IsInherited) {
    assert(Cand2IsInherited);
    auto *Cand1Class = cast<CXXRecordDecl>(Cand1.Function->getDeclContext());
    auto *Cand2Class = cast<CXXRecordDecl>(Cand2.Function->getDeclContext());
    if (Cand1Class->isDerivedFrom(Cand2Class))
      return true;
    if (Cand2Class->isDerivedFrom(Cand1Class))
      return false;
    // Inherited from sibling base classes: still ambiguous.
  }

  //   -- F2 is a rewritten candidate (12.4.1.2) and F1 is not
  //   -- F1 and F2 are rewritten candidates, and F2 is a synthesized candidate
  //      with reversed order of parameters and F1 is not
  //
  // We rank reversed + different operator as worse than just reversed, but
  // that comparison can never happen, because we only consider reversing for
  // the maximally-rewritten operator (== or <=>).
  if (Cand1.RewriteKind != Cand2.RewriteKind)
    return Cand1.RewriteKind < Cand2.RewriteKind;

  // Check C++17 tie-breakers for deduction guides.
  {
    auto *Guide1 = dyn_cast_or_null<CXXDeductionGuideDecl>(Cand1.Function);
    auto *Guide2 = dyn_cast_or_null<CXXDeductionGuideDecl>(Cand2.Function);
    if (Guide1 && Guide2) {
      //  -- F1 is generated from a deduction-guide and F2 is not
      if (Guide1->isImplicit() != Guide2->isImplicit())
        return Guide2->isImplicit();

      //  -- F1 is the copy deduction candidate(16.3.1.8) and F2 is not
      if (Guide1->isCopyDeductionCandidate())
        return true;
    }
  }

  // Check for enable_if value-based overload resolution.
  if (Cand1.Function && Cand2.Function) {
    Comparison Cmp = compareEnableIfAttrs(S, Cand1.Function, Cand2.Function);
    if (Cmp != Comparison::Equal)
      return Cmp == Comparison::Better;
  }

  bool HasPS1 = Cand1.Function != nullptr &&
                functionHasPassObjectSizeParams(Cand1.Function);
  bool HasPS2 = Cand2.Function != nullptr &&
                functionHasPassObjectSizeParams(Cand2.Function);
  if (HasPS1 != HasPS2 && HasPS1)
    return true;

  auto MV = isBetterMultiversionCandidate(Cand1, Cand2);
  if (MV == Comparison::Better)
    return true;
  if (MV == Comparison::Worse)
    return false;

  // If other rules cannot determine which is better, CUDA preference is used
  // to determine which is better.
  if (S.getLangOpts().CUDA && Cand1.Function && Cand2.Function) {
    FunctionDecl *Caller = S.getCurFunctionDecl(/*AllowLambda=*/true);
    return S.IdentifyCUDAPreference(Caller, Cand1.Function) >
           S.IdentifyCUDAPreference(Caller, Cand2.Function);
  }

  // General member function overloading is handled above, so this only handles
  // constructors with address spaces.
  // This only handles address spaces since C++ has no other
  // qualifier that can be used with constructors.
  const auto *CD1 = dyn_cast_or_null<CXXConstructorDecl>(Cand1.Function);
  const auto *CD2 = dyn_cast_or_null<CXXConstructorDecl>(Cand2.Function);
  if (CD1 && CD2) {
    LangAS AS1 = CD1->getMethodQualifiers().getAddressSpace();
    LangAS AS2 = CD2->getMethodQualifiers().getAddressSpace();
    if (AS1 != AS2) {
      if (Qualifiers::isAddressSpaceSupersetOf(AS2, AS1))
        return true;
      if (Qualifiers::isAddressSpaceSupersetOf(AS2, AS1))
        return false;
    }
  }

  return false;
}

/// Determine whether two declarations are "equivalent" for the purposes of
/// name lookup and overload resolution. This applies when the same internal/no
/// linkage entity is defined by two modules (probably by textually including
/// the same header). In such a case, we don't consider the declarations to
/// declare the same entity, but we also don't want lookups with both
/// declarations visible to be ambiguous in some cases (this happens when using
/// a modularized libstdc++).
bool Sema::isEquivalentInternalLinkageDeclaration(const NamedDecl *A,
                                                  const NamedDecl *B) {
  auto *VA = dyn_cast_or_null<ValueDecl>(A);
  auto *VB = dyn_cast_or_null<ValueDecl>(B);
  if (!VA || !VB)
    return false;

  // The declarations must be declaring the same name as an internal linkage
  // entity in different modules.
  if (!VA->getDeclContext()->getRedeclContext()->Equals(
          VB->getDeclContext()->getRedeclContext()) ||
      getOwningModule(VA) == getOwningModule(VB) ||
      VA->isExternallyVisible() || VB->isExternallyVisible())
    return false;

  // Check that the declarations appear to be equivalent.
  //
  // FIXME: Checking the type isn't really enough to resolve the ambiguity.
  // For constants and functions, we should check the initializer or body is
  // the same. For non-constant variables, we shouldn't allow it at all.
  if (Context.hasSameType(VA->getType(), VB->getType()))
    return true;

  // Enum constants within unnamed enumerations will have different types, but
  // may still be similar enough to be interchangeable for our purposes.
  if (auto *EA = dyn_cast<EnumConstantDecl>(VA)) {
    if (auto *EB = dyn_cast<EnumConstantDecl>(VB)) {
      // Only handle anonymous enums. If the enumerations were named and
      // equivalent, they would have been merged to the same type.
      auto *EnumA = cast<EnumDecl>(EA->getDeclContext());
      auto *EnumB = cast<EnumDecl>(EB->getDeclContext());
      if (EnumA->hasNameForLinkage() || EnumB->hasNameForLinkage() ||
          !Context.hasSameType(EnumA->getIntegerType(),
                               EnumB->getIntegerType()))
        return false;
      // Allow this only if the value is the same for both enumerators.
      return llvm::APSInt::isSameValue(EA->getInitVal(), EB->getInitVal());
    }
  }

  // Nothing else is sufficiently similar.
  return false;
}

void Sema::diagnoseEquivalentInternalLinkageDeclarations(
    SourceLocation Loc, const NamedDecl *D, ArrayRef<const NamedDecl *> Equiv) {
  assert(D && "Unknown declaration");
  Diag(Loc, diag::ext_equivalent_internal_linkage_decl_in_modules) << D;

  Module *M = getOwningModule(D);
  Diag(D->getLocation(), diag::note_equivalent_internal_linkage_decl)
      << !M << (M ? M->getFullModuleName() : "");

  for (auto *E : Equiv) {
    Module *M = getOwningModule(E);
    Diag(E->getLocation(), diag::note_equivalent_internal_linkage_decl)
        << !M << (M ? M->getFullModuleName() : "");
  }
}

bool OverloadCandidate::NotValidBecauseConstraintExprHasError() const {
  return FailureKind == ovl_fail_bad_deduction &&
         DeductionFailure.Result == Sema::TDK_ConstraintsNotSatisfied &&
         static_cast<CNSInfo *>(DeductionFailure.Data)
             ->Satisfaction.ContainsErrors;
}

/// Computes the best viable function (C++ 13.3.3)
/// within an overload candidate set.
///
/// \param Loc The location of the function name (or operator symbol) for
/// which overload resolution occurs.
///
/// \param Best If overload resolution was successful or found a deleted
/// function, \p Best points to the candidate function found.
///
/// \returns The result of overload resolution.
OverloadingResult
OverloadCandidateSet::BestViableFunction(Sema &S, SourceLocation Loc,
                                         iterator &Best) {
  llvm::SmallVector<OverloadCandidate *, 16> Candidates;
  std::transform(begin(), end(), std::back_inserter(Candidates),
                 [](OverloadCandidate &Cand) { return &Cand; });

  // [CUDA] HD->H or HD->D calls are technically not allowed by CUDA but
  // are accepted by both clang and NVCC. However, during a particular
  // compilation mode only one call variant is viable. We need to
  // exclude non-viable overload candidates from consideration based
  // only on their host/device attributes. Specifically, if one
  // candidate call is WrongSide and the other is SameSide, we ignore
  // the WrongSide candidate.
  // We only need to remove wrong-sided candidates here if
  // -fgpu-exclude-wrong-side-overloads is off. When
  // -fgpu-exclude-wrong-side-overloads is on, all candidates are compared
  // uniformly in isBetterOverloadCandidate.
  if (S.getLangOpts().CUDA && !S.getLangOpts().GPUExcludeWrongSideOverloads) {
    const FunctionDecl *Caller = S.getCurFunctionDecl(/*AllowLambda=*/true);
    bool ContainsSameSideCandidate =
        llvm::any_of(Candidates, [&](OverloadCandidate *Cand) {
          // Check viable function only.
          return Cand->Viable && Cand->Function &&
                 S.IdentifyCUDAPreference(Caller, Cand->Function) ==
                     Sema::CFP_SameSide;
        });
    if (ContainsSameSideCandidate) {
      auto IsWrongSideCandidate = [&](OverloadCandidate *Cand) {
        // Check viable function only to avoid unnecessary data copying/moving.
        return Cand->Viable && Cand->Function &&
               S.IdentifyCUDAPreference(Caller, Cand->Function) ==
                   Sema::CFP_WrongSide;
      };
      llvm::erase_if(Candidates, IsWrongSideCandidate);
    }
  }

  // Find the best viable function.
  Best = end();
  for (auto *Cand : Candidates) {
    Cand->Best = false;
    if (Cand->Viable) {
      if (Best == end() ||
          isBetterOverloadCandidate(S, *Cand, *Best, Loc, Kind))
        Best = Cand;
    } else if (Cand->NotValidBecauseConstraintExprHasError()) {
      // This candidate has constraint that we were unable to evaluate because
      // it referenced an expression that contained an error. Rather than fall
      // back onto a potentially unintended candidate (made worse by
      // subsuming constraints), treat this as 'no viable candidate'.
      Best = end();
      return OR_No_Viable_Function;
    }
  }

  // If we didn't find any viable functions, abort.
  if (Best == end())
    return OR_No_Viable_Function;

  llvm::SmallVector<const NamedDecl *, 4> EquivalentCands;

  llvm::SmallVector<OverloadCandidate*, 4> PendingBest;
  PendingBest.push_back(&*Best);
  Best->Best = true;

  // Make sure that this function is better than every other viable
  // function. If not, we have an ambiguity.
  while (!PendingBest.empty()) {
    auto *Curr = PendingBest.pop_back_val();
    for (auto *Cand : Candidates) {
      if (Cand->Viable && !Cand->Best &&
          !isBetterOverloadCandidate(S, *Curr, *Cand, Loc, Kind)) {
        PendingBest.push_back(Cand);
        Cand->Best = true;

        if (S.isEquivalentInternalLinkageDeclaration(Cand->Function,
                                                     Curr->Function))
          EquivalentCands.push_back(Cand->Function);
        else
          Best = end();
      }
    }
  }

  // If we found more than one best candidate, this is ambiguous.
  if (Best == end())
    return OR_Ambiguous;

  // Best is the best viable function.
  if (Best->Function && Best->Function->isDeleted())
    return OR_Deleted;

  if (!EquivalentCands.empty())
    S.diagnoseEquivalentInternalLinkageDeclarations(Loc, Best->Function,
                                                    EquivalentCands);

  return OR_Success;
}

namespace {

enum OverloadCandidateKind {
  oc_function,
  oc_method,
  oc_reversed_binary_operator,
  oc_constructor,
  oc_implicit_default_constructor,
  oc_implicit_copy_constructor,
  oc_implicit_move_constructor,
  oc_implicit_copy_assignment,
  oc_implicit_move_assignment,
  oc_implicit_equality_comparison,
  oc_inherited_constructor
};

enum OverloadCandidateSelect {
  ocs_non_template,
  ocs_template,
  ocs_described_template,
};

static std::pair<OverloadCandidateKind, OverloadCandidateSelect>
ClassifyOverloadCandidate(Sema &S, const NamedDecl *Found,
                          const FunctionDecl *Fn,
                          OverloadCandidateRewriteKind CRK,
                          std::string &Description) {

  bool isTemplate = Fn->isTemplateDecl() || Found->isTemplateDecl();
  if (FunctionTemplateDecl *FunTmpl = Fn->getPrimaryTemplate()) {
    isTemplate = true;
    Description = S.getTemplateArgumentBindingsText(
        FunTmpl->getTemplateParameters(), *Fn->getTemplateSpecializationArgs());
  }

  OverloadCandidateSelect Select = [&]() {
    if (!Description.empty())
      return ocs_described_template;
    return isTemplate ? ocs_template : ocs_non_template;
  }();

  OverloadCandidateKind Kind = [&]() {
    if (Fn->isImplicit() && Fn->getOverloadedOperator() == OO_EqualEqual)
      return oc_implicit_equality_comparison;

    if (CRK & CRK_Reversed)
      return oc_reversed_binary_operator;

    if (const auto *Ctor = dyn_cast<CXXConstructorDecl>(Fn)) {
      if (!Ctor->isImplicit()) {
        if (isa<ConstructorUsingShadowDecl>(Found))
          return oc_inherited_constructor;
        else
          return oc_constructor;
      }

      if (Ctor->isDefaultConstructor())
        return oc_implicit_default_constructor;

      if (Ctor->isMoveConstructor())
        return oc_implicit_move_constructor;

      assert(Ctor->isCopyConstructor() &&
             "unexpected sort of implicit constructor");
      return oc_implicit_copy_constructor;
    }

    if (const auto *Meth = dyn_cast<CXXMethodDecl>(Fn)) {
      // This actually gets spelled 'candidate function' for now, but
      // it doesn't hurt to split it out.
      if (!Meth->isImplicit())
        return oc_method;

      if (Meth->isMoveAssignmentOperator())
        return oc_implicit_move_assignment;

      if (Meth->isCopyAssignmentOperator())
        return oc_implicit_copy_assignment;

      assert(isa<CXXConversionDecl>(Meth) && "expected conversion");
      return oc_method;
    }

    return oc_function;
  }();

  return std::make_pair(Kind, Select);
}

void MaybeEmitInheritedConstructorNote(Sema &S, const Decl *FoundDecl) {
  // FIXME: It'd be nice to only emit a note once per using-decl per overload
  // set.
  if (const auto *Shadow = dyn_cast<ConstructorUsingShadowDecl>(FoundDecl))
    S.Diag(FoundDecl->getLocation(),
           diag::note_ovl_candidate_inherited_constructor)
      << Shadow->getNominatedBaseClass();
}

} // end anonymous namespace

static bool isFunctionAlwaysEnabled(const ASTContext &Ctx,
                                    const FunctionDecl *FD) {
  for (auto *EnableIf : FD->specific_attrs<EnableIfAttr>()) {
    bool AlwaysTrue;
    if (EnableIf->getCond()->isValueDependent() ||
        !EnableIf->getCond()->EvaluateAsBooleanCondition(AlwaysTrue, Ctx))
      return false;
    if (!AlwaysTrue)
      return false;
  }
  return true;
}

/// Returns true if we can take the address of the function.
///
/// \param Complain - If true, we'll emit a diagnostic
/// \param InOverloadResolution - For the purposes of emitting a diagnostic, are
///   we in overload resolution?
/// \param Loc - The location of the statement we're complaining about. Ignored
///   if we're not complaining, or if we're in overload resolution.
static bool checkAddressOfFunctionIsAvailable(Sema &S, const FunctionDecl *FD,
                                              bool Complain,
                                              bool InOverloadResolution,
                                              SourceLocation Loc) {
  if (!isFunctionAlwaysEnabled(S.Context, FD)) {
    if (Complain) {
      if (InOverloadResolution)
        S.Diag(FD->getBeginLoc(),
               diag::note_addrof_ovl_candidate_disabled_by_enable_if_attr);
      else
        S.Diag(Loc, diag::err_addrof_function_disabled_by_enable_if_attr) << FD;
    }
    return false;
  }

  if (FD->getTrailingRequiresClause()) {
    ConstraintSatisfaction Satisfaction;
    if (S.CheckFunctionConstraints(FD, Satisfaction, Loc))
      return false;
    if (!Satisfaction.IsSatisfied) {
      if (Complain) {
        if (InOverloadResolution) {
          SmallString<128> TemplateArgString;
          if (FunctionTemplateDecl *FunTmpl = FD->getPrimaryTemplate()) {
            TemplateArgString += " ";
            TemplateArgString += S.getTemplateArgumentBindingsText(
                FunTmpl->getTemplateParameters(),
                *FD->getTemplateSpecializationArgs());
          }

          S.Diag(FD->getBeginLoc(),
                 diag::note_ovl_candidate_unsatisfied_constraints)
              << TemplateArgString;
        } else
          S.Diag(Loc, diag::err_addrof_function_constraints_not_satisfied)
              << FD;
        S.DiagnoseUnsatisfiedConstraint(Satisfaction);
      }
      return false;
    }
  }

  auto I = llvm::find_if(FD->parameters(), [](const ParmVarDecl *P) {
    return P->hasAttr<PassObjectSizeAttr>();
  });
  if (I == FD->param_end())
    return true;

  if (Complain) {
    // Add one to ParamNo because it's user-facing
    unsigned ParamNo = std::distance(FD->param_begin(), I) + 1;
    if (InOverloadResolution)
      S.Diag(FD->getLocation(),
             diag::note_ovl_candidate_has_pass_object_size_params)
          << ParamNo;
    else
      S.Diag(Loc, diag::err_address_of_function_with_pass_object_size_params)
          << FD << ParamNo;
  }
  return false;
}

static bool checkAddressOfCandidateIsAvailable(Sema &S,
                                               const FunctionDecl *FD) {
  return checkAddressOfFunctionIsAvailable(S, FD, /*Complain=*/true,
                                           /*InOverloadResolution=*/true,
                                           /*Loc=*/SourceLocation());
}

bool Sema::checkAddressOfFunctionIsAvailable(const FunctionDecl *Function,
                                             bool Complain,
                                             SourceLocation Loc) {
  return ::checkAddressOfFunctionIsAvailable(*this, Function, Complain,
                                             /*InOverloadResolution=*/false,
                                             Loc);
}

// Don't print candidates other than the one that matches the calling
// convention of the call operator, since that is guaranteed to exist.
static bool shouldSkipNotingLambdaConversionDecl(const FunctionDecl *Fn) {
  const auto *ConvD = dyn_cast<CXXConversionDecl>(Fn);

  if (!ConvD)
    return false;
  const auto *RD = cast<CXXRecordDecl>(Fn->getParent());
  if (!RD->isLambda())
    return false;

  CXXMethodDecl *CallOp = RD->getLambdaCallOperator();
  CallingConv CallOpCC =
      CallOp->getType()->castAs<FunctionType>()->getCallConv();
  QualType ConvRTy = ConvD->getType()->castAs<FunctionType>()->getReturnType();
  CallingConv ConvToCC =
      ConvRTy->getPointeeType()->castAs<FunctionType>()->getCallConv();

  return ConvToCC != CallOpCC;
}

// Notes the location of an overload candidate.
void Sema::NoteOverloadCandidate(const NamedDecl *Found, const FunctionDecl *Fn,
                                 OverloadCandidateRewriteKind RewriteKind,
                                 QualType DestType, bool TakingAddress) {
  if (TakingAddress && !checkAddressOfCandidateIsAvailable(*this, Fn))
    return;
  if (Fn->isMultiVersion() && Fn->hasAttr<TargetAttr>() &&
      !Fn->getAttr<TargetAttr>()->isDefaultVersion())
    return;
  if (Fn->isMultiVersion() && Fn->hasAttr<TargetVersionAttr>() &&
      !Fn->getAttr<TargetVersionAttr>()->isDefaultVersion())
    return;
  if (shouldSkipNotingLambdaConversionDecl(Fn))
    return;

  std::string FnDesc;
  std::pair<OverloadCandidateKind, OverloadCandidateSelect> KSPair =
      ClassifyOverloadCandidate(*this, Found, Fn, RewriteKind, FnDesc);
  PartialDiagnostic PD = PDiag(diag::note_ovl_candidate)
                         << (unsigned)KSPair.first << (unsigned)KSPair.second
                         << Fn << FnDesc;

  HandleFunctionTypeMismatch(PD, Fn->getType(), DestType);
  Diag(Fn->getLocation(), PD);
  MaybeEmitInheritedConstructorNote(*this, Found);
}

static void
MaybeDiagnoseAmbiguousConstraints(Sema &S, ArrayRef<OverloadCandidate> Cands) {
  // Perhaps the ambiguity was caused by two atomic constraints that are
  // 'identical' but not equivalent:
  //
  // void foo() requires (sizeof(T) > 4) { } // #1
  // void foo() requires (sizeof(T) > 4) && T::value { } // #2
  //
  // The 'sizeof(T) > 4' constraints are seemingly equivalent and should cause
  // #2 to subsume #1, but these constraint are not considered equivalent
  // according to the subsumption rules because they are not the same
  // source-level construct. This behavior is quite confusing and we should try
  // to help the user figure out what happened.

  SmallVector<const Expr *, 3> FirstAC, SecondAC;
  FunctionDecl *FirstCand = nullptr, *SecondCand = nullptr;
  for (auto I = Cands.begin(), E = Cands.end(); I != E; ++I) {
    if (!I->Function)
      continue;
    SmallVector<const Expr *, 3> AC;
    if (auto *Template = I->Function->getPrimaryTemplate())
      Template->getAssociatedConstraints(AC);
    else
      I->Function->getAssociatedConstraints(AC);
    if (AC.empty())
      continue;
    if (FirstCand == nullptr) {
      FirstCand = I->Function;
      FirstAC = AC;
    } else if (SecondCand == nullptr) {
      SecondCand = I->Function;
      SecondAC = AC;
    } else {
      // We have more than one pair of constrained functions - this check is
      // expensive and we'd rather not try to diagnose it.
      return;
    }
  }
  if (!SecondCand)
    return;
  // The diagnostic can only happen if there are associated constraints on
  // both sides (there needs to be some identical atomic constraint).
  if (S.MaybeEmitAmbiguousAtomicConstraintsDiagnostic(FirstCand, FirstAC,
                                                      SecondCand, SecondAC))
    // Just show the user one diagnostic, they'll probably figure it out
    // from here.
    return;
}

// Notes the location of all overload candidates designated through
// OverloadedExpr
void Sema::NoteAllOverloadCandidates(Expr *OverloadedExpr, QualType DestType,
                                     bool TakingAddress) {
  assert(OverloadedExpr->getType() == Context.OverloadTy);

  OverloadExpr::FindResult Ovl = OverloadExpr::find(OverloadedExpr);
  OverloadExpr *OvlExpr = Ovl.Expression;

  for (UnresolvedSetIterator I = OvlExpr->decls_begin(),
                            IEnd = OvlExpr->decls_end();
       I != IEnd; ++I) {
    if (FunctionTemplateDecl *FunTmpl =
                dyn_cast<FunctionTemplateDecl>((*I)->getUnderlyingDecl()) ) {
      NoteOverloadCandidate(*I, FunTmpl->getTemplatedDecl(), CRK_None, DestType,
                            TakingAddress);
    } else if (FunctionDecl *Fun
                      = dyn_cast<FunctionDecl>((*I)->getUnderlyingDecl()) ) {
      NoteOverloadCandidate(*I, Fun, CRK_None, DestType, TakingAddress);
    }
  }
}

/// Diagnoses an ambiguous conversion.  The partial diagnostic is the
/// "lead" diagnostic; it will be given two arguments, the source and
/// target types of the conversion.
void ImplicitConversionSequence::DiagnoseAmbiguousConversion(
                                 Sema &S,
                                 SourceLocation CaretLoc,
                                 const PartialDiagnostic &PDiag) const {
  S.Diag(CaretLoc, PDiag)
    << Ambiguous.getFromType() << Ambiguous.getToType();
  unsigned CandsShown = 0;
  AmbiguousConversionSequence::const_iterator I, E;
  for (I = Ambiguous.begin(), E = Ambiguous.end(); I != E; ++I) {
    if (CandsShown >= S.Diags.getNumOverloadCandidatesToShow())
      break;
    ++CandsShown;
    S.NoteOverloadCandidate(I->first, I->second);
  }
  S.Diags.overloadCandidatesShown(CandsShown);
  if (I != E)
    S.Diag(SourceLocation(), diag::note_ovl_too_many_candidates) << int(E - I);
}

static void DiagnoseBadConversion(Sema &S, OverloadCandidate *Cand,
                                  unsigned I, bool TakingCandidateAddress) {
  const ImplicitConversionSequence &Conv = Cand->Conversions[I];
  assert(Conv.isBad());
  assert(Cand->Function && "for now, candidate must be a function");
  FunctionDecl *Fn = Cand->Function;

  // There's a conversion slot for the object argument if this is a
  // non-constructor method.  Note that 'I' corresponds the
  // conversion-slot index.
  bool isObjectArgument = false;
  if (isa<CXXMethodDecl>(Fn) && !isa<CXXConstructorDecl>(Fn)) {
    if (I == 0)
      isObjectArgument = true;
    else
      I--;
  }

  std::string FnDesc;
  std::pair<OverloadCandidateKind, OverloadCandidateSelect> FnKindPair =
      ClassifyOverloadCandidate(S, Cand->FoundDecl, Fn, Cand->getRewriteKind(),
                                FnDesc);

  Expr *FromExpr = Conv.Bad.FromExpr;
  QualType FromTy = Conv.Bad.getFromType();
  QualType ToTy = Conv.Bad.getToType();

  if (FromTy == S.Context.OverloadTy) {
    assert(FromExpr && "overload set argument came from implicit argument?");
    Expr *E = FromExpr->IgnoreParens();
    if (isa<UnaryOperator>(E))
      E = cast<UnaryOperator>(E)->getSubExpr()->IgnoreParens();
    DeclarationName Name = cast<OverloadExpr>(E)->getName();

    S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_overload)
        << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
        << (FromExpr ? FromExpr->getSourceRange() : SourceRange()) << ToTy
        << Name << I + 1;
    MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
    return;
  }

  // Do some hand-waving analysis to see if the non-viability is due
  // to a qualifier mismatch.
  CanQualType CFromTy = S.Context.getCanonicalType(FromTy);
  CanQualType CToTy = S.Context.getCanonicalType(ToTy);
  if (CanQual<ReferenceType> RT = CToTy->getAs<ReferenceType>())
    CToTy = RT->getPointeeType();
  else {
    // TODO: detect and diagnose the full richness of const mismatches.
    if (CanQual<PointerType> FromPT = CFromTy->getAs<PointerType>())
      if (CanQual<PointerType> ToPT = CToTy->getAs<PointerType>()) {
        CFromTy = FromPT->getPointeeType();
        CToTy = ToPT->getPointeeType();
      }
  }

  if (CToTy.getUnqualifiedType() == CFromTy.getUnqualifiedType() &&
      !CToTy.isAtLeastAsQualifiedAs(CFromTy)) {
    Qualifiers FromQs = CFromTy.getQualifiers();
    Qualifiers ToQs = CToTy.getQualifiers();

    if (FromQs.getAddressSpace() != ToQs.getAddressSpace()) {
      if (isObjectArgument)
        S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_addrspace_this)
            << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second
            << FnDesc << (FromExpr ? FromExpr->getSourceRange() : SourceRange())
            << FromQs.getAddressSpace() << ToQs.getAddressSpace();
      else
        S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_addrspace)
            << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second
            << FnDesc << (FromExpr ? FromExpr->getSourceRange() : SourceRange())
            << FromQs.getAddressSpace() << ToQs.getAddressSpace()
            << ToTy->isReferenceType() << I + 1;
      MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
      return;
    }

    if (FromQs.getObjCLifetime() != ToQs.getObjCLifetime()) {
      S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_ownership)
          << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
          << (FromExpr ? FromExpr->getSourceRange() : SourceRange()) << FromTy
          << FromQs.getObjCLifetime() << ToQs.getObjCLifetime()
          << (unsigned)isObjectArgument << I + 1;
      MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
      return;
    }

    if (FromQs.getObjCGCAttr() != ToQs.getObjCGCAttr()) {
      S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_gc)
          << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
          << (FromExpr ? FromExpr->getSourceRange() : SourceRange()) << FromTy
          << FromQs.getObjCGCAttr() << ToQs.getObjCGCAttr()
          << (unsigned)isObjectArgument << I + 1;
      MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
      return;
    }

    if (FromQs.hasUnaligned() != ToQs.hasUnaligned()) {
      S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_unaligned)
          << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
          << (FromExpr ? FromExpr->getSourceRange() : SourceRange()) << FromTy
          << FromQs.hasUnaligned() << I + 1;
      MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
      return;
    }

    unsigned CVR = FromQs.getCVRQualifiers() & ~ToQs.getCVRQualifiers();
    assert(CVR && "expected qualifiers mismatch");

    if (isObjectArgument) {
      S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_cvr_this)
          << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
          << (FromExpr ? FromExpr->getSourceRange() : SourceRange()) << FromTy
          << (CVR - 1);
    } else {
      S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_cvr)
          << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
          << (FromExpr ? FromExpr->getSourceRange() : SourceRange()) << FromTy
          << (CVR - 1) << I + 1;
    }
    MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
    return;
  }

  if (Conv.Bad.Kind == BadConversionSequence::lvalue_ref_to_rvalue ||
      Conv.Bad.Kind == BadConversionSequence::rvalue_ref_to_lvalue) {
    S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_value_category)
        << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
        << (unsigned)isObjectArgument << I + 1
        << (Conv.Bad.Kind == BadConversionSequence::rvalue_ref_to_lvalue)
        << (FromExpr ? FromExpr->getSourceRange() : SourceRange());
    MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
    return;
  }

  // Special diagnostic for failure to convert an initializer list, since
  // telling the user that it has type void is not useful.
  if (FromExpr && isa<InitListExpr>(FromExpr)) {
    S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_list_argument)
        << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
        << (FromExpr ? FromExpr->getSourceRange() : SourceRange()) << FromTy
        << ToTy << (unsigned)isObjectArgument << I + 1
        << (Conv.Bad.Kind == BadConversionSequence::too_few_initializers ? 1
            : Conv.Bad.Kind == BadConversionSequence::too_many_initializers
                ? 2
                : 0);
    MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
    return;
  }

  // Diagnose references or pointers to incomplete types differently,
  // since it's far from impossible that the incompleteness triggered
  // the failure.
  QualType TempFromTy = FromTy.getNonReferenceType();
  if (const PointerType *PTy = TempFromTy->getAs<PointerType>())
    TempFromTy = PTy->getPointeeType();
  if (TempFromTy->isIncompleteType()) {
    // Emit the generic diagnostic and, optionally, add the hints to it.
    S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_conv_incomplete)
        << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
        << (FromExpr ? FromExpr->getSourceRange() : SourceRange()) << FromTy
        << ToTy << (unsigned)isObjectArgument << I + 1
        << (unsigned)(Cand->Fix.Kind);

    MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
    return;
  }

  // Diagnose base -> derived pointer conversions.
  unsigned BaseToDerivedConversion = 0;
  if (const PointerType *FromPtrTy = FromTy->getAs<PointerType>()) {
    if (const PointerType *ToPtrTy = ToTy->getAs<PointerType>()) {
      if (ToPtrTy->getPointeeType().isAtLeastAsQualifiedAs(
                                               FromPtrTy->getPointeeType()) &&
          !FromPtrTy->getPointeeType()->isIncompleteType() &&
          !ToPtrTy->getPointeeType()->isIncompleteType() &&
          S.IsDerivedFrom(SourceLocation(), ToPtrTy->getPointeeType(),
                          FromPtrTy->getPointeeType()))
        BaseToDerivedConversion = 1;
    }
  } else if (const ObjCObjectPointerType *FromPtrTy
                                    = FromTy->getAs<ObjCObjectPointerType>()) {
    if (const ObjCObjectPointerType *ToPtrTy
                                        = ToTy->getAs<ObjCObjectPointerType>())
      if (const ObjCInterfaceDecl *FromIface = FromPtrTy->getInterfaceDecl())
        if (const ObjCInterfaceDecl *ToIface = ToPtrTy->getInterfaceDecl())
          if (ToPtrTy->getPointeeType().isAtLeastAsQualifiedAs(
                                                FromPtrTy->getPointeeType()) &&
              FromIface->isSuperClassOf(ToIface))
            BaseToDerivedConversion = 2;
  } else if (const ReferenceType *ToRefTy = ToTy->getAs<ReferenceType>()) {
    if (ToRefTy->getPointeeType().isAtLeastAsQualifiedAs(FromTy) &&
        !FromTy->isIncompleteType() &&
        !ToRefTy->getPointeeType()->isIncompleteType() &&
        S.IsDerivedFrom(SourceLocation(), ToRefTy->getPointeeType(), FromTy)) {
      BaseToDerivedConversion = 3;
    }
  }

  if (BaseToDerivedConversion) {
    S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_base_to_derived_conv)
        << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
        << (FromExpr ? FromExpr->getSourceRange() : SourceRange())
        << (BaseToDerivedConversion - 1) << FromTy << ToTy << I + 1;
    MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
    return;
  }

  if (isa<ObjCObjectPointerType>(CFromTy) &&
      isa<PointerType>(CToTy)) {
      Qualifiers FromQs = CFromTy.getQualifiers();
      Qualifiers ToQs = CToTy.getQualifiers();
      if (FromQs.getObjCLifetime() != ToQs.getObjCLifetime()) {
        S.Diag(Fn->getLocation(), diag::note_ovl_candidate_bad_arc_conv)
            << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second
            << FnDesc << (FromExpr ? FromExpr->getSourceRange() : SourceRange())
            << FromTy << ToTy << (unsigned)isObjectArgument << I + 1;
        MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
        return;
      }
  }

  if (TakingCandidateAddress &&
      !checkAddressOfCandidateIsAvailable(S, Cand->Function))
    return;

  // Emit the generic diagnostic and, optionally, add the hints to it.
  PartialDiagnostic FDiag = S.PDiag(diag::note_ovl_candidate_bad_conv);
  FDiag << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
        << (FromExpr ? FromExpr->getSourceRange() : SourceRange()) << FromTy
        << ToTy << (unsigned)isObjectArgument << I + 1
        << (unsigned)(Cand->Fix.Kind);

  // Check that location of Fn is not in system header.
  if (!S.SourceMgr.isInSystemHeader(Fn->getLocation())) {
    // If we can fix the conversion, suggest the FixIts.
    for (const FixItHint &HI : Cand->Fix.Hints)
        FDiag << HI;
  }

  S.Diag(Fn->getLocation(), FDiag);

  MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
}

/// Additional arity mismatch diagnosis specific to a function overload
/// candidates. This is not covered by the more general DiagnoseArityMismatch()
/// over a candidate in any candidate set.
static bool CheckArityMismatch(Sema &S, OverloadCandidate *Cand,
                               unsigned NumArgs) {
  FunctionDecl *Fn = Cand->Function;
  unsigned MinParams = Fn->getMinRequiredArguments();

  // With invalid overloaded operators, it's possible that we think we
  // have an arity mismatch when in fact it looks like we have the
  // right number of arguments, because only overloaded operators have
  // the weird behavior of overloading member and non-member functions.
  // Just don't report anything.
  if (Fn->isInvalidDecl() &&
      Fn->getDeclName().getNameKind() == DeclarationName::CXXOperatorName)
    return true;

  if (NumArgs < MinParams) {
    assert((Cand->FailureKind == ovl_fail_too_few_arguments) ||
           (Cand->FailureKind == ovl_fail_bad_deduction &&
            Cand->DeductionFailure.Result == Sema::TDK_TooFewArguments));
  } else {
    assert((Cand->FailureKind == ovl_fail_too_many_arguments) ||
           (Cand->FailureKind == ovl_fail_bad_deduction &&
            Cand->DeductionFailure.Result == Sema::TDK_TooManyArguments));
  }

  return false;
}

/// General arity mismatch diagnosis over a candidate in a candidate set.
static void DiagnoseArityMismatch(Sema &S, NamedDecl *Found, Decl *D,
                                  unsigned NumFormalArgs) {
  assert(isa<FunctionDecl>(D) &&
      "The templated declaration should at least be a function"
      " when diagnosing bad template argument deduction due to too many"
      " or too few arguments");

  FunctionDecl *Fn = cast<FunctionDecl>(D);

  // TODO: treat calls to a missing default constructor as a special case
  const auto *FnTy = Fn->getType()->castAs<FunctionProtoType>();
  unsigned MinParams = Fn->getMinRequiredArguments();

  // at least / at most / exactly
  unsigned mode, modeCount;
  if (NumFormalArgs < MinParams) {
    if (MinParams != FnTy->getNumParams() || FnTy->isVariadic() ||
        FnTy->isTemplateVariadic())
      mode = 0; // "at least"
    else
      mode = 2; // "exactly"
    modeCount = MinParams;
  } else {
    if (MinParams != FnTy->getNumParams())
      mode = 1; // "at most"
    else
      mode = 2; // "exactly"
    modeCount = FnTy->getNumParams();
  }

  std::string Description;
  std::pair<OverloadCandidateKind, OverloadCandidateSelect> FnKindPair =
      ClassifyOverloadCandidate(S, Found, Fn, CRK_None, Description);

  if (modeCount == 1 && Fn->getParamDecl(0)->getDeclName())
    S.Diag(Fn->getLocation(), diag::note_ovl_candidate_arity_one)
        << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second
        << Description << mode << Fn->getParamDecl(0) << NumFormalArgs;
  else
    S.Diag(Fn->getLocation(), diag::note_ovl_candidate_arity)
        << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second
        << Description << mode << modeCount << NumFormalArgs;

  MaybeEmitInheritedConstructorNote(S, Found);
}

/// Arity mismatch diagnosis specific to a function overload candidate.
static void DiagnoseArityMismatch(Sema &S, OverloadCandidate *Cand,
                                  unsigned NumFormalArgs) {
  if (!CheckArityMismatch(S, Cand, NumFormalArgs))
    DiagnoseArityMismatch(S, Cand->FoundDecl, Cand->Function, NumFormalArgs);
}

static TemplateDecl *getDescribedTemplate(Decl *Templated) {
  if (TemplateDecl *TD = Templated->getDescribedTemplate())
    return TD;
  llvm_unreachable("Unsupported: Getting the described template declaration"
                   " for bad deduction diagnosis");
}

/// Diagnose a failed template-argument deduction.
static void DiagnoseBadDeduction(Sema &S, NamedDecl *Found, Decl *Templated,
                                 DeductionFailureInfo &DeductionFailure,
                                 unsigned NumArgs,
                                 bool TakingCandidateAddress) {
  TemplateParameter Param = DeductionFailure.getTemplateParameter();
  NamedDecl *ParamD;
  (ParamD = Param.dyn_cast<TemplateTypeParmDecl*>()) ||
  (ParamD = Param.dyn_cast<NonTypeTemplateParmDecl*>()) ||
  (ParamD = Param.dyn_cast<TemplateTemplateParmDecl*>());
  switch (DeductionFailure.Result) {
  case Sema::TDK_Success:
    llvm_unreachable("TDK_success while diagnosing bad deduction");

  case Sema::TDK_Incomplete: {
    assert(ParamD && "no parameter found for incomplete deduction result");
    S.Diag(Templated->getLocation(),
           diag::note_ovl_candidate_incomplete_deduction)
        << ParamD->getDeclName();
    MaybeEmitInheritedConstructorNote(S, Found);
    return;
  }

  case Sema::TDK_IncompletePack: {
    assert(ParamD && "no parameter found for incomplete deduction result");
    S.Diag(Templated->getLocation(),
           diag::note_ovl_candidate_incomplete_deduction_pack)
        << ParamD->getDeclName()
        << (DeductionFailure.getFirstArg()->pack_size() + 1)
        << *DeductionFailure.getFirstArg();
    MaybeEmitInheritedConstructorNote(S, Found);
    return;
  }

  case Sema::TDK_Underqualified: {
    assert(ParamD && "no parameter found for bad qualifiers deduction result");
    TemplateTypeParmDecl *TParam = cast<TemplateTypeParmDecl>(ParamD);

    QualType Param = DeductionFailure.getFirstArg()->getAsType();

    // Param will have been canonicalized, but it should just be a
    // qualified version of ParamD, so move the qualifiers to that.
    QualifierCollector Qs;
    Qs.strip(Param);
    QualType NonCanonParam = Qs.apply(S.Context, TParam->getTypeForDecl());
    assert(S.Context.hasSameType(Param, NonCanonParam));

    // Arg has also been canonicalized, but there's nothing we can do
    // about that.  It also doesn't matter as much, because it won't
    // have any template parameters in it (because deduction isn't
    // done on dependent types).
    QualType Arg = DeductionFailure.getSecondArg()->getAsType();

    S.Diag(Templated->getLocation(), diag::note_ovl_candidate_underqualified)
        << ParamD->getDeclName() << Arg << NonCanonParam;
    MaybeEmitInheritedConstructorNote(S, Found);
    return;
  }

  case Sema::TDK_Inconsistent: {
    assert(ParamD && "no parameter found for inconsistent deduction result");
    int which = 0;
    if (isa<TemplateTypeParmDecl>(ParamD))
      which = 0;
    else if (isa<NonTypeTemplateParmDecl>(ParamD)) {
      // Deduction might have failed because we deduced arguments of two
      // different types for a non-type template parameter.
      // FIXME: Use a different TDK value for this.
      QualType T1 =
          DeductionFailure.getFirstArg()->getNonTypeTemplateArgumentType();
      QualType T2 =
          DeductionFailure.getSecondArg()->getNonTypeTemplateArgumentType();
      if (!T1.isNull() && !T2.isNull() && !S.Context.hasSameType(T1, T2)) {
        S.Diag(Templated->getLocation(),
               diag::note_ovl_candidate_inconsistent_deduction_types)
          << ParamD->getDeclName() << *DeductionFailure.getFirstArg() << T1
          << *DeductionFailure.getSecondArg() << T2;
        MaybeEmitInheritedConstructorNote(S, Found);
        return;
      }

      which = 1;
    } else {
      which = 2;
    }

    // Tweak the diagnostic if the problem is that we deduced packs of
    // different arities. We'll print the actual packs anyway in case that
    // includes additional useful information.
    if (DeductionFailure.getFirstArg()->getKind() == TemplateArgument::Pack &&
        DeductionFailure.getSecondArg()->getKind() == TemplateArgument::Pack &&
        DeductionFailure.getFirstArg()->pack_size() !=
            DeductionFailure.getSecondArg()->pack_size()) {
      which = 3;
    }

    S.Diag(Templated->getLocation(),
           diag::note_ovl_candidate_inconsistent_deduction)
        << which << ParamD->getDeclName() << *DeductionFailure.getFirstArg()
        << *DeductionFailure.getSecondArg();
    MaybeEmitInheritedConstructorNote(S, Found);
    return;
  }

  case Sema::TDK_InvalidExplicitArguments:
    assert(ParamD && "no parameter found for invalid explicit arguments");
    if (ParamD->getDeclName())
      S.Diag(Templated->getLocation(),
             diag::note_ovl_candidate_explicit_arg_mismatch_named)
          << ParamD->getDeclName();
    else {
      int index = 0;
      if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(ParamD))
        index = TTP->getIndex();
      else if (NonTypeTemplateParmDecl *NTTP
                                  = dyn_cast<NonTypeTemplateParmDecl>(ParamD))
        index = NTTP->getIndex();
      else
        index = cast<TemplateTemplateParmDecl>(ParamD)->getIndex();
      S.Diag(Templated->getLocation(),
             diag::note_ovl_candidate_explicit_arg_mismatch_unnamed)
          << (index + 1);
    }
    MaybeEmitInheritedConstructorNote(S, Found);
    return;

  case Sema::TDK_ConstraintsNotSatisfied: {
    // Format the template argument list into the argument string.
    SmallString<128> TemplateArgString;
    TemplateArgumentList *Args = DeductionFailure.getTemplateArgumentList();
    TemplateArgString = " ";
    TemplateArgString += S.getTemplateArgumentBindingsText(
        getDescribedTemplate(Templated)->getTemplateParameters(), *Args);
    if (TemplateArgString.size() == 1)
      TemplateArgString.clear();
    S.Diag(Templated->getLocation(),
           diag::note_ovl_candidate_unsatisfied_constraints)
        << TemplateArgString;

    S.DiagnoseUnsatisfiedConstraint(
        static_cast<CNSInfo*>(DeductionFailure.Data)->Satisfaction);
    return;
  }
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
    DiagnoseArityMismatch(S, Found, Templated, NumArgs);
    return;

  case Sema::TDK_InstantiationDepth:
    S.Diag(Templated->getLocation(),
           diag::note_ovl_candidate_instantiation_depth);
    MaybeEmitInheritedConstructorNote(S, Found);
    return;

  case Sema::TDK_SubstitutionFailure: {
    // Format the template argument list into the argument string.
    SmallString<128> TemplateArgString;
    if (TemplateArgumentList *Args =
            DeductionFailure.getTemplateArgumentList()) {
      TemplateArgString = " ";
      TemplateArgString += S.getTemplateArgumentBindingsText(
          getDescribedTemplate(Templated)->getTemplateParameters(), *Args);
      if (TemplateArgString.size() == 1)
        TemplateArgString.clear();
    }

    // If this candidate was disabled by enable_if, say so.
    PartialDiagnosticAt *PDiag = DeductionFailure.getSFINAEDiagnostic();
    if (PDiag && PDiag->second.getDiagID() ==
          diag::err_typename_nested_not_found_enable_if) {
      // FIXME: Use the source range of the condition, and the fully-qualified
      //        name of the enable_if template. These are both present in PDiag.
      S.Diag(PDiag->first, diag::note_ovl_candidate_disabled_by_enable_if)
        << "'enable_if'" << TemplateArgString;
      return;
    }

    // We found a specific requirement that disabled the enable_if.
    if (PDiag && PDiag->second.getDiagID() ==
        diag::err_typename_nested_not_found_requirement) {
      S.Diag(Templated->getLocation(),
             diag::note_ovl_candidate_disabled_by_requirement)
        << PDiag->second.getStringArg(0) << TemplateArgString;
      return;
    }

    // Format the SFINAE diagnostic into the argument string.
    // FIXME: Add a general mechanism to include a PartialDiagnostic *'s
    //        formatted message in another diagnostic.
    SmallString<128> SFINAEArgString;
    SourceRange R;
    if (PDiag) {
      SFINAEArgString = ": ";
      R = SourceRange(PDiag->first, PDiag->first);
      PDiag->second.EmitToString(S.getDiagnostics(), SFINAEArgString);
    }

    S.Diag(Templated->getLocation(),
           diag::note_ovl_candidate_substitution_failure)
        << TemplateArgString << SFINAEArgString << R;
    MaybeEmitInheritedConstructorNote(S, Found);
    return;
  }

  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested: {
    // Format the template argument list into the argument string.
    SmallString<128> TemplateArgString;
    if (TemplateArgumentList *Args =
            DeductionFailure.getTemplateArgumentList()) {
      TemplateArgString = " ";
      TemplateArgString += S.getTemplateArgumentBindingsText(
          getDescribedTemplate(Templated)->getTemplateParameters(), *Args);
      if (TemplateArgString.size() == 1)
        TemplateArgString.clear();
    }

    S.Diag(Templated->getLocation(), diag::note_ovl_candidate_deduced_mismatch)
        << (*DeductionFailure.getCallArgIndex() + 1)
        << *DeductionFailure.getFirstArg() << *DeductionFailure.getSecondArg()
        << TemplateArgString
        << (DeductionFailure.Result == Sema::TDK_DeducedMismatchNested);
    break;
  }

  case Sema::TDK_NonDeducedMismatch: {
    // FIXME: Provide a source location to indicate what we couldn't match.
    TemplateArgument FirstTA = *DeductionFailure.getFirstArg();
    TemplateArgument SecondTA = *DeductionFailure.getSecondArg();
    if (FirstTA.getKind() == TemplateArgument::Template &&
        SecondTA.getKind() == TemplateArgument::Template) {
      TemplateName FirstTN = FirstTA.getAsTemplate();
      TemplateName SecondTN = SecondTA.getAsTemplate();
      if (FirstTN.getKind() == TemplateName::Template &&
          SecondTN.getKind() == TemplateName::Template) {
        if (FirstTN.getAsTemplateDecl()->getName() ==
            SecondTN.getAsTemplateDecl()->getName()) {
          // FIXME: This fixes a bad diagnostic where both templates are named
          // the same.  This particular case is a bit difficult since:
          // 1) It is passed as a string to the diagnostic printer.
          // 2) The diagnostic printer only attempts to find a better
          //    name for types, not decls.
          // Ideally, this should folded into the diagnostic printer.
          S.Diag(Templated->getLocation(),
                 diag::note_ovl_candidate_non_deduced_mismatch_qualified)
              << FirstTN.getAsTemplateDecl() << SecondTN.getAsTemplateDecl();
          return;
        }
      }
    }

    if (TakingCandidateAddress && isa<FunctionDecl>(Templated) &&
        !checkAddressOfCandidateIsAvailable(S, cast<FunctionDecl>(Templated)))
      return;

    // FIXME: For generic lambda parameters, check if the function is a lambda
    // call operator, and if so, emit a prettier and more informative
    // diagnostic that mentions 'auto' and lambda in addition to
    // (or instead of?) the canonical template type parameters.
    S.Diag(Templated->getLocation(),
           diag::note_ovl_candidate_non_deduced_mismatch)
        << FirstTA << SecondTA;
    return;
  }
  // TODO: diagnose these individually, then kill off
  // note_ovl_candidate_bad_deduction, which is uselessly vague.
  case Sema::TDK_MiscellaneousDeductionFailure:
    S.Diag(Templated->getLocation(), diag::note_ovl_candidate_bad_deduction);
    MaybeEmitInheritedConstructorNote(S, Found);
    return;
  case Sema::TDK_CUDATargetMismatch:
    S.Diag(Templated->getLocation(),
           diag::note_cuda_ovl_candidate_target_mismatch);
    return;
  }
}

/// Diagnose a failed template-argument deduction, for function calls.
static void DiagnoseBadDeduction(Sema &S, OverloadCandidate *Cand,
                                 unsigned NumArgs,
                                 bool TakingCandidateAddress) {
  unsigned TDK = Cand->DeductionFailure.Result;
  if (TDK == Sema::TDK_TooFewArguments || TDK == Sema::TDK_TooManyArguments) {
    if (CheckArityMismatch(S, Cand, NumArgs))
      return;
  }
  DiagnoseBadDeduction(S, Cand->FoundDecl, Cand->Function, // pattern
                       Cand->DeductionFailure, NumArgs, TakingCandidateAddress);
}

/// CUDA: diagnose an invalid call across targets.
static void DiagnoseBadTarget(Sema &S, OverloadCandidate *Cand) {
  FunctionDecl *Caller = S.getCurFunctionDecl(/*AllowLambda=*/true);
  FunctionDecl *Callee = Cand->Function;

  Sema::CUDAFunctionTarget CallerTarget = S.IdentifyCUDATarget(Caller),
                           CalleeTarget = S.IdentifyCUDATarget(Callee);

  std::string FnDesc;
  std::pair<OverloadCandidateKind, OverloadCandidateSelect> FnKindPair =
      ClassifyOverloadCandidate(S, Cand->FoundDecl, Callee,
                                Cand->getRewriteKind(), FnDesc);

  S.Diag(Callee->getLocation(), diag::note_ovl_candidate_bad_target)
      << (unsigned)FnKindPair.first << (unsigned)ocs_non_template
      << FnDesc /* Ignored */
      << CalleeTarget << CallerTarget;

  // This could be an implicit constructor for which we could not infer the
  // target due to a collsion. Diagnose that case.
  CXXMethodDecl *Meth = dyn_cast<CXXMethodDecl>(Callee);
  if (Meth != nullptr && Meth->isImplicit()) {
    CXXRecordDecl *ParentClass = Meth->getParent();
    Sema::CXXSpecialMember CSM;

    switch (FnKindPair.first) {
    default:
      return;
    case oc_implicit_default_constructor:
      CSM = Sema::CXXDefaultConstructor;
      break;
    case oc_implicit_copy_constructor:
      CSM = Sema::CXXCopyConstructor;
      break;
    case oc_implicit_move_constructor:
      CSM = Sema::CXXMoveConstructor;
      break;
    case oc_implicit_copy_assignment:
      CSM = Sema::CXXCopyAssignment;
      break;
    case oc_implicit_move_assignment:
      CSM = Sema::CXXMoveAssignment;
      break;
    };

    bool ConstRHS = false;
    if (Meth->getNumParams()) {
      if (const ReferenceType *RT =
              Meth->getParamDecl(0)->getType()->getAs<ReferenceType>()) {
        ConstRHS = RT->getPointeeType().isConstQualified();
      }
    }

    S.inferCUDATargetForImplicitSpecialMember(ParentClass, CSM, Meth,
                                              /* ConstRHS */ ConstRHS,
                                              /* Diagnose */ true);
  }
}

static void DiagnoseFailedEnableIfAttr(Sema &S, OverloadCandidate *Cand) {
  FunctionDecl *Callee = Cand->Function;
  EnableIfAttr *Attr = static_cast<EnableIfAttr*>(Cand->DeductionFailure.Data);

  S.Diag(Callee->getLocation(),
         diag::note_ovl_candidate_disabled_by_function_cond_attr)
      << Attr->getCond()->getSourceRange() << Attr->getMessage();
}

static void DiagnoseFailedExplicitSpec(Sema &S, OverloadCandidate *Cand) {
  ExplicitSpecifier ES = ExplicitSpecifier::getFromDecl(Cand->Function);
  assert(ES.isExplicit() && "not an explicit candidate");

  unsigned Kind;
  switch (Cand->Function->getDeclKind()) {
  case Decl::Kind::CXXConstructor:
    Kind = 0;
    break;
  case Decl::Kind::CXXConversion:
    Kind = 1;
    break;
  case Decl::Kind::CXXDeductionGuide:
    Kind = Cand->Function->isImplicit() ? 0 : 2;
    break;
  default:
    llvm_unreachable("invalid Decl");
  }

  // Note the location of the first (in-class) declaration; a redeclaration
  // (particularly an out-of-class definition) will typically lack the
  // 'explicit' specifier.
  // FIXME: This is probably a good thing to do for all 'candidate' notes.
  FunctionDecl *First = Cand->Function->getFirstDecl();
  if (FunctionDecl *Pattern = First->getTemplateInstantiationPattern())
    First = Pattern->getFirstDecl();

  S.Diag(First->getLocation(),
         diag::note_ovl_candidate_explicit)
      << Kind << (ES.getExpr() ? 1 : 0)
      << (ES.getExpr() ? ES.getExpr()->getSourceRange() : SourceRange());
}

/// Generates a 'note' diagnostic for an overload candidate.  We've
/// already generated a primary error at the call site.
///
/// It really does need to be a single diagnostic with its caret
/// pointed at the candidate declaration.  Yes, this creates some
/// major challenges of technical writing.  Yes, this makes pointing
/// out problems with specific arguments quite awkward.  It's still
/// better than generating twenty screens of text for every failed
/// overload.
///
/// It would be great to be able to express per-candidate problems
/// more richly for those diagnostic clients that cared, but we'd
/// still have to be just as careful with the default diagnostics.
/// \param CtorDestAS Addr space of object being constructed (for ctor
/// candidates only).
static void NoteFunctionCandidate(Sema &S, OverloadCandidate *Cand,
                                  unsigned NumArgs,
                                  bool TakingCandidateAddress,
                                  LangAS CtorDestAS = LangAS::Default) {
  FunctionDecl *Fn = Cand->Function;
  if (shouldSkipNotingLambdaConversionDecl(Fn))
    return;

  // There is no physical candidate declaration to point to for OpenCL builtins.
  // Except for failed conversions, the notes are identical for each candidate,
  // so do not generate such notes.
  if (S.getLangOpts().OpenCL && Fn->isImplicit() &&
      Cand->FailureKind != ovl_fail_bad_conversion)
    return;

  // Note deleted candidates, but only if they're viable.
  if (Cand->Viable) {
    if (Fn->isDeleted()) {
      std::string FnDesc;
      std::pair<OverloadCandidateKind, OverloadCandidateSelect> FnKindPair =
          ClassifyOverloadCandidate(S, Cand->FoundDecl, Fn,
                                    Cand->getRewriteKind(), FnDesc);

      S.Diag(Fn->getLocation(), diag::note_ovl_candidate_deleted)
          << (unsigned)FnKindPair.first << (unsigned)FnKindPair.second << FnDesc
          << (Fn->isDeleted() ? (Fn->isDeletedAsWritten() ? 1 : 2) : 0);
      MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
      return;
    }

    // We don't really have anything else to say about viable candidates.
    S.NoteOverloadCandidate(Cand->FoundDecl, Fn, Cand->getRewriteKind());
    return;
  }

  switch (Cand->FailureKind) {
  case ovl_fail_too_many_arguments:
  case ovl_fail_too_few_arguments:
    return DiagnoseArityMismatch(S, Cand, NumArgs);

  case ovl_fail_bad_deduction:
    return DiagnoseBadDeduction(S, Cand, NumArgs,
                                TakingCandidateAddress);

  case ovl_fail_illegal_constructor: {
    S.Diag(Fn->getLocation(), diag::note_ovl_candidate_illegal_constructor)
      << (Fn->getPrimaryTemplate() ? 1 : 0);
    MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
    return;
  }

  case ovl_fail_object_addrspace_mismatch: {
    Qualifiers QualsForPrinting;
    QualsForPrinting.setAddressSpace(CtorDestAS);
    S.Diag(Fn->getLocation(),
           diag::note_ovl_candidate_illegal_constructor_adrspace_mismatch)
        << QualsForPrinting;
    MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
    return;
  }

  case ovl_fail_trivial_conversion:
  case ovl_fail_bad_final_conversion:
  case ovl_fail_final_conversion_not_exact:
    return S.NoteOverloadCandidate(Cand->FoundDecl, Fn, Cand->getRewriteKind());

  case ovl_fail_bad_conversion: {
    unsigned I = (Cand->IgnoreObjectArgument ? 1 : 0);
    for (unsigned N = Cand->Conversions.size(); I != N; ++I)
      if (Cand->Conversions[I].isBad())
        return DiagnoseBadConversion(S, Cand, I, TakingCandidateAddress);

    // FIXME: this currently happens when we're called from SemaInit
    // when user-conversion overload fails.  Figure out how to handle
    // those conditions and diagnose them well.
    return S.NoteOverloadCandidate(Cand->FoundDecl, Fn, Cand->getRewriteKind());
  }

  case ovl_fail_bad_target:
    return DiagnoseBadTarget(S, Cand);

  case ovl_fail_enable_if:
    return DiagnoseFailedEnableIfAttr(S, Cand);

  case ovl_fail_explicit:
    return DiagnoseFailedExplicitSpec(S, Cand);

  case ovl_fail_inhctor_slice:
    // It's generally not interesting to note copy/move constructors here.
    if (cast<CXXConstructorDecl>(Fn)->isCopyOrMoveConstructor())
      return;
    S.Diag(Fn->getLocation(),
           diag::note_ovl_candidate_inherited_constructor_slice)
      << (Fn->getPrimaryTemplate() ? 1 : 0)
      << Fn->getParamDecl(0)->getType()->isRValueReferenceType();
    MaybeEmitInheritedConstructorNote(S, Cand->FoundDecl);
    return;

  case ovl_fail_addr_not_available: {
    bool Available = checkAddressOfCandidateIsAvailable(S, Cand->Function);
    (void)Available;
    assert(!Available);
    break;
  }
  case ovl_non_default_multiversion_function:
    // Do nothing, these should simply be ignored.
    break;

  case ovl_fail_constraints_not_satisfied: {
    std::string FnDesc;
    std::pair<OverloadCandidateKind, OverloadCandidateSelect> FnKindPair =
        ClassifyOverloadCandidate(S, Cand->FoundDecl, Fn,
                                  Cand->getRewriteKind(), FnDesc);

    S.Diag(Fn->getLocation(),
           diag::note_ovl_candidate_constraints_not_satisfied)
        << (unsigned)FnKindPair.first << (unsigned)ocs_non_template
        << FnDesc /* Ignored */;
    ConstraintSatisfaction Satisfaction;
    if (S.CheckFunctionConstraints(Fn, Satisfaction))
      break;
    S.DiagnoseUnsatisfiedConstraint(Satisfaction);
  }
  }
}

static void NoteSurrogateCandidate(Sema &S, OverloadCandidate *Cand) {
  if (shouldSkipNotingLambdaConversionDecl(Cand->Surrogate))
    return;

  // Desugar the type of the surrogate down to a function type,
  // retaining as many typedefs as possible while still showing
  // the function type (and, therefore, its parameter types).
  QualType FnType = Cand->Surrogate->getConversionType();
  bool isLValueReference = false;
  bool isRValueReference = false;
  bool isPointer = false;
  if (const LValueReferenceType *FnTypeRef =
        FnType->getAs<LValueReferenceType>()) {
    FnType = FnTypeRef->getPointeeType();
    isLValueReference = true;
  } else if (const RValueReferenceType *FnTypeRef =
               FnType->getAs<RValueReferenceType>()) {
    FnType = FnTypeRef->getPointeeType();
    isRValueReference = true;
  }
  if (const PointerType *FnTypePtr = FnType->getAs<PointerType>()) {
    FnType = FnTypePtr->getPointeeType();
    isPointer = true;
  }
  // Desugar down to a function type.
  FnType = QualType(FnType->getAs<FunctionType>(), 0);
  // Reconstruct the pointer/reference as appropriate.
  if (isPointer) FnType = S.Context.getPointerType(FnType);
  if (isRValueReference) FnType = S.Context.getRValueReferenceType(FnType);
  if (isLValueReference) FnType = S.Context.getLValueReferenceType(FnType);

  S.Diag(Cand->Surrogate->getLocation(), diag::note_ovl_surrogate_cand)
    << FnType;
}

static void NoteBuiltinOperatorCandidate(Sema &S, StringRef Opc,
                                         SourceLocation OpLoc,
                                         OverloadCandidate *Cand) {
  assert(Cand->Conversions.size() <= 2 && "builtin operator is not binary");
  std::string TypeStr("operator");
  TypeStr += Opc;
  TypeStr += "(";
  TypeStr += Cand->BuiltinParamTypes[0].getAsString();
  if (Cand->Conversions.size() == 1) {
    TypeStr += ")";
    S.Diag(OpLoc, diag::note_ovl_builtin_candidate) << TypeStr;
  } else {
    TypeStr += ", ";
    TypeStr += Cand->BuiltinParamTypes[1].getAsString();
    TypeStr += ")";
    S.Diag(OpLoc, diag::note_ovl_builtin_candidate) << TypeStr;
  }
}

static void NoteAmbiguousUserConversions(Sema &S, SourceLocation OpLoc,
                                         OverloadCandidate *Cand) {
  for (const ImplicitConversionSequence &ICS : Cand->Conversions) {
    if (ICS.isBad()) break; // all meaningless after first invalid
    if (!ICS.isAmbiguous()) continue;

    ICS.DiagnoseAmbiguousConversion(
        S, OpLoc, S.PDiag(diag::note_ambiguous_type_conversion));
  }
}

static SourceLocation GetLocationForCandidate(const OverloadCandidate *Cand) {
  if (Cand->Function)
    return Cand->Function->getLocation();
  if (Cand->IsSurrogate)
    return Cand->Surrogate->getLocation();
  return SourceLocation();
}

static unsigned RankDeductionFailure(const DeductionFailureInfo &DFI) {
  switch ((Sema::TemplateDeductionResult)DFI.Result) {
  case Sema::TDK_Success:
  case Sema::TDK_NonDependentConversionFailure:
  case Sema::TDK_AlreadyDiagnosed:
    llvm_unreachable("non-deduction failure while diagnosing bad deduction");

  case Sema::TDK_Invalid:
  case Sema::TDK_Incomplete:
  case Sema::TDK_IncompletePack:
    return 1;

  case Sema::TDK_Underqualified:
  case Sema::TDK_Inconsistent:
    return 2;

  case Sema::TDK_SubstitutionFailure:
  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_ConstraintsNotSatisfied:
  case Sema::TDK_DeducedMismatchNested:
  case Sema::TDK_NonDeducedMismatch:
  case Sema::TDK_MiscellaneousDeductionFailure:
  case Sema::TDK_CUDATargetMismatch:
    return 3;

  case Sema::TDK_InstantiationDepth:
    return 4;

  case Sema::TDK_InvalidExplicitArguments:
    return 5;

  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
    return 6;
  }
  llvm_unreachable("Unhandled deduction result");
}

namespace {
struct CompareOverloadCandidatesForDisplay {
  Sema &S;
  SourceLocation Loc;
  size_t NumArgs;
  OverloadCandidateSet::CandidateSetKind CSK;

  CompareOverloadCandidatesForDisplay(
      Sema &S, SourceLocation Loc, size_t NArgs,
      OverloadCandidateSet::CandidateSetKind CSK)
      : S(S), NumArgs(NArgs), CSK(CSK) {}

  OverloadFailureKind EffectiveFailureKind(const OverloadCandidate *C) const {
    // If there are too many or too few arguments, that's the high-order bit we
    // want to sort by, even if the immediate failure kind was something else.
    if (C->FailureKind == ovl_fail_too_many_arguments ||
        C->FailureKind == ovl_fail_too_few_arguments)
      return static_cast<OverloadFailureKind>(C->FailureKind);

    if (C->Function) {
      if (NumArgs > C->Function->getNumParams() && !C->Function->isVariadic())
        return ovl_fail_too_many_arguments;
      if (NumArgs < C->Function->getMinRequiredArguments())
        return ovl_fail_too_few_arguments;
    }

    return static_cast<OverloadFailureKind>(C->FailureKind);
  }

  bool operator()(const OverloadCandidate *L,
                  const OverloadCandidate *R) {
    // Fast-path this check.
    if (L == R) return false;

    // Order first by viability.
    if (L->Viable) {
      if (!R->Viable) return true;

      // TODO: introduce a tri-valued comparison for overload
      // candidates.  Would be more worthwhile if we had a sort
      // that could exploit it.
      if (isBetterOverloadCandidate(S, *L, *R, SourceLocation(), CSK))
        return true;
      if (isBetterOverloadCandidate(S, *R, *L, SourceLocation(), CSK))
        return false;
    } else if (R->Viable)
      return false;

    assert(L->Viable == R->Viable);

    // Criteria by which we can sort non-viable candidates:
    if (!L->Viable) {
      OverloadFailureKind LFailureKind = EffectiveFailureKind(L);
      OverloadFailureKind RFailureKind = EffectiveFailureKind(R);

      // 1. Arity mismatches come after other candidates.
      if (LFailureKind == ovl_fail_too_many_arguments ||
          LFailureKind == ovl_fail_too_few_arguments) {
        if (RFailureKind == ovl_fail_too_many_arguments ||
            RFailureKind == ovl_fail_too_few_arguments) {
          int LDist = std::abs((int)L->getNumParams() - (int)NumArgs);
          int RDist = std::abs((int)R->getNumParams() - (int)NumArgs);
          if (LDist == RDist) {
            if (LFailureKind == RFailureKind)
              // Sort non-surrogates before surrogates.
              return !L->IsSurrogate && R->IsSurrogate;
            // Sort candidates requiring fewer parameters than there were
            // arguments given after candidates requiring more parameters
            // than there were arguments given.
            return LFailureKind == ovl_fail_too_many_arguments;
          }
          return LDist < RDist;
        }
        return false;
      }
      if (RFailureKind == ovl_fail_too_many_arguments ||
          RFailureKind == ovl_fail_too_few_arguments)
        return true;

      // 2. Bad conversions come first and are ordered by the number
      // of bad conversions and quality of good conversions.
      if (LFailureKind == ovl_fail_bad_conversion) {
        if (RFailureKind != ovl_fail_bad_conversion)
          return true;

        // The conversion that can be fixed with a smaller number of changes,
        // comes first.
        unsigned numLFixes = L->Fix.NumConversionsFixed;
        unsigned numRFixes = R->Fix.NumConversionsFixed;
        numLFixes = (numLFixes == 0) ? UINT_MAX : numLFixes;
        numRFixes = (numRFixes == 0) ? UINT_MAX : numRFixes;
        if (numLFixes != numRFixes) {
          return numLFixes < numRFixes;
        }

        // If there's any ordering between the defined conversions...
        // FIXME: this might not be transitive.
        assert(L->Conversions.size() == R->Conversions.size());

        int leftBetter = 0;
        unsigned I = (L->IgnoreObjectArgument || R->IgnoreObjectArgument);
        for (unsigned E = L->Conversions.size(); I != E; ++I) {
          switch (CompareImplicitConversionSequences(S, Loc,
                                                     L->Conversions[I],
                                                     R->Conversions[I])) {
          case ImplicitConversionSequence::Better:
            leftBetter++;
            break;

          case ImplicitConversionSequence::Worse:
            leftBetter--;
            break;

          case ImplicitConversionSequence::Indistinguishable:
            break;
          }
        }
        if (leftBetter > 0) return true;
        if (leftBetter < 0) return false;

      } else if (RFailureKind == ovl_fail_bad_conversion)
        return false;

      if (LFailureKind == ovl_fail_bad_deduction) {
        if (RFailureKind != ovl_fail_bad_deduction)
          return true;

        if (L->DeductionFailure.Result != R->DeductionFailure.Result)
          return RankDeductionFailure(L->DeductionFailure)
               < RankDeductionFailure(R->DeductionFailure);
      } else if (RFailureKind == ovl_fail_bad_deduction)
        return false;

      // TODO: others?
    }

    // Sort everything else by location.
    SourceLocation LLoc = GetLocationForCandidate(L);
    SourceLocation RLoc = GetLocationForCandidate(R);

    // Put candidates without locations (e.g. builtins) at the end.
    if (LLoc.isInvalid()) return false;
    if (RLoc.isInvalid()) return true;

    return S.SourceMgr.isBeforeInTranslationUnit(LLoc, RLoc);
  }
};
}

/// CompleteNonViableCandidate - Normally, overload resolution only
/// computes up to the first bad conversion. Produces the FixIt set if
/// possible.
static void
CompleteNonViableCandidate(Sema &S, OverloadCandidate *Cand,
                           ArrayRef<Expr *> Args,
                           OverloadCandidateSet::CandidateSetKind CSK) {
  assert(!Cand->Viable);

  // Don't do anything on failures other than bad conversion.
  if (Cand->FailureKind != ovl_fail_bad_conversion)
    return;

  // We only want the FixIts if all the arguments can be corrected.
  bool Unfixable = false;
  // Use a implicit copy initialization to check conversion fixes.
  Cand->Fix.setConversionChecker(TryCopyInitialization);

  // Attempt to fix the bad conversion.
  unsigned ConvCount = Cand->Conversions.size();
  for (unsigned ConvIdx = (Cand->IgnoreObjectArgument ? 1 : 0); /**/;
       ++ConvIdx) {
    assert(ConvIdx != ConvCount && "no bad conversion in candidate");
    if (Cand->Conversions[ConvIdx].isInitialized() &&
        Cand->Conversions[ConvIdx].isBad()) {
      Unfixable = !Cand->TryToFixBadConversion(ConvIdx, S);
      break;
    }
  }

  // FIXME: this should probably be preserved from the overload
  // operation somehow.
  bool SuppressUserConversions = false;

  unsigned ConvIdx = 0;
  unsigned ArgIdx = 0;
  ArrayRef<QualType> ParamTypes;
  bool Reversed = Cand->isReversed();

  if (Cand->IsSurrogate) {
    QualType ConvType
      = Cand->Surrogate->getConversionType().getNonReferenceType();
    if (const PointerType *ConvPtrType = ConvType->getAs<PointerType>())
      ConvType = ConvPtrType->getPointeeType();
    ParamTypes = ConvType->castAs<FunctionProtoType>()->getParamTypes();
    // Conversion 0 is 'this', which doesn't have a corresponding parameter.
    ConvIdx = 1;
  } else if (Cand->Function) {
    ParamTypes =
        Cand->Function->getType()->castAs<FunctionProtoType>()->getParamTypes();
    if (isa<CXXMethodDecl>(Cand->Function) &&
        !isa<CXXConstructorDecl>(Cand->Function) && !Reversed) {
      // Conversion 0 is 'this', which doesn't have a corresponding parameter.
      ConvIdx = 1;
      if (CSK == OverloadCandidateSet::CSK_Operator &&
          Cand->Function->getDeclName().getCXXOverloadedOperator() != OO_Call &&
          Cand->Function->getDeclName().getCXXOverloadedOperator() !=
              OO_Subscript)
        // Argument 0 is 'this', which doesn't have a corresponding parameter.
        ArgIdx = 1;
    }
  } else {
    // Builtin operator.
    assert(ConvCount <= 3);
    ParamTypes = Cand->BuiltinParamTypes;
  }

  // Fill in the rest of the conversions.
  for (unsigned ParamIdx = Reversed ? ParamTypes.size() - 1 : 0;
       ConvIdx != ConvCount;
       ++ConvIdx, ++ArgIdx, ParamIdx += (Reversed ? -1 : 1)) {
    assert(ArgIdx < Args.size() && "no argument for this arg conversion");
    if (Cand->Conversions[ConvIdx].isInitialized()) {
      // We've already checked this conversion.
    } else if (ParamIdx < ParamTypes.size()) {
      if (ParamTypes[ParamIdx]->isDependentType())
        Cand->Conversions[ConvIdx].setAsIdentityConversion(
            Args[ArgIdx]->getType());
      else {
        Cand->Conversions[ConvIdx] =
            TryCopyInitialization(S, Args[ArgIdx], ParamTypes[ParamIdx],
                                  SuppressUserConversions,
                                  /*InOverloadResolution=*/true,
                                  /*AllowObjCWritebackConversion=*/
                                  S.getLangOpts().ObjCAutoRefCount);
        // Store the FixIt in the candidate if it exists.
        if (!Unfixable && Cand->Conversions[ConvIdx].isBad())
          Unfixable = !Cand->TryToFixBadConversion(ConvIdx, S);
      }
    } else
      Cand->Conversions[ConvIdx].setEllipsis();
  }
}

SmallVector<OverloadCandidate *, 32> OverloadCandidateSet::CompleteCandidates(
    Sema &S, OverloadCandidateDisplayKind OCD, ArrayRef<Expr *> Args,
    SourceLocation OpLoc,
    llvm::function_ref<bool(OverloadCandidate &)> Filter) {
  // Sort the candidates by viability and position.  Sorting directly would
  // be prohibitive, so we make a set of pointers and sort those.
  SmallVector<OverloadCandidate*, 32> Cands;
  if (OCD == OCD_AllCandidates) Cands.reserve(size());
  for (iterator Cand = begin(), LastCand = end(); Cand != LastCand; ++Cand) {
    if (!Filter(*Cand))
      continue;
    switch (OCD) {
    case OCD_AllCandidates:
      if (!Cand->Viable) {
        if (!Cand->Function && !Cand->IsSurrogate) {
          // This a non-viable builtin candidate.  We do not, in general,
          // want to list every possible builtin candidate.
          continue;
        }
        CompleteNonViableCandidate(S, Cand, Args, Kind);
      }
      break;

    case OCD_ViableCandidates:
      if (!Cand->Viable)
        continue;
      break;

    case OCD_AmbiguousCandidates:
      if (!Cand->Best)
        continue;
      break;
    }

    Cands.push_back(Cand);
  }

  llvm::stable_sort(
      Cands, CompareOverloadCandidatesForDisplay(S, OpLoc, Args.size(), Kind));

  return Cands;
}

bool OverloadCandidateSet::shouldDeferDiags(Sema &S, ArrayRef<Expr *> Args,
                                            SourceLocation OpLoc) {
  bool DeferHint = false;
  if (S.getLangOpts().CUDA && S.getLangOpts().GPUDeferDiag) {
    // Defer diagnostic for CUDA/HIP if there are wrong-sided candidates or
    // host device candidates.
    auto WrongSidedCands =
        CompleteCandidates(S, OCD_AllCandidates, Args, OpLoc, [](auto &Cand) {
          return (Cand.Viable == false &&
                  Cand.FailureKind == ovl_fail_bad_target) ||
                 (Cand.Function &&
                  Cand.Function->template hasAttr<CUDAHostAttr>() &&
                  Cand.Function->template hasAttr<CUDADeviceAttr>());
        });
    DeferHint = !WrongSidedCands.empty();
  }
  return DeferHint;
}

/// When overload resolution fails, prints diagnostic messages containing the
/// candidates in the candidate set.
void OverloadCandidateSet::NoteCandidates(
    PartialDiagnosticAt PD, Sema &S, OverloadCandidateDisplayKind OCD,
    ArrayRef<Expr *> Args, StringRef Opc, SourceLocation OpLoc,
    llvm::function_ref<bool(OverloadCandidate &)> Filter) {

  auto Cands = CompleteCandidates(S, OCD, Args, OpLoc, Filter);

  S.Diag(PD.first, PD.second, shouldDeferDiags(S, Args, OpLoc));

  NoteCandidates(S, Args, Cands, Opc, OpLoc);

  if (OCD == OCD_AmbiguousCandidates)
    MaybeDiagnoseAmbiguousConstraints(S, {begin(), end()});
}

void OverloadCandidateSet::NoteCandidates(Sema &S, ArrayRef<Expr *> Args,
                                          ArrayRef<OverloadCandidate *> Cands,
                                          StringRef Opc, SourceLocation OpLoc) {
  bool ReportedAmbiguousConversions = false;

  const OverloadsShown ShowOverloads = S.Diags.getShowOverloads();
  unsigned CandsShown = 0;
  auto I = Cands.begin(), E = Cands.end();
  for (; I != E; ++I) {
    OverloadCandidate *Cand = *I;

    if (CandsShown >= S.Diags.getNumOverloadCandidatesToShow() &&
        ShowOverloads == Ovl_Best) {
      break;
    }
    ++CandsShown;

    if (Cand->Function)
      NoteFunctionCandidate(S, Cand, Args.size(),
                            /*TakingCandidateAddress=*/false, DestAS);
    else if (Cand->IsSurrogate)
      NoteSurrogateCandidate(S, Cand);
    else {
      assert(Cand->Viable &&
             "Non-viable built-in candidates are not added to Cands.");
      // Generally we only see ambiguities including viable builtin
      // operators if overload resolution got screwed up by an
      // ambiguous user-defined conversion.
      //
      // FIXME: It's quite possible for different conversions to see
      // different ambiguities, though.
      if (!ReportedAmbiguousConversions) {
        NoteAmbiguousUserConversions(S, OpLoc, Cand);
        ReportedAmbiguousConversions = true;
      }

      // If this is a viable builtin, print it.
      NoteBuiltinOperatorCandidate(S, Opc, OpLoc, Cand);
    }
  }

  // Inform S.Diags that we've shown an overload set with N elements.  This may
  // inform the future value of S.Diags.getNumOverloadCandidatesToShow().
  S.Diags.overloadCandidatesShown(CandsShown);

  if (I != E)
    S.Diag(OpLoc, diag::note_ovl_too_many_candidates,
           shouldDeferDiags(S, Args, OpLoc))
        << int(E - I);
}

static SourceLocation
GetLocationForCandidate(const TemplateSpecCandidate *Cand) {
  return Cand->Specialization ? Cand->Specialization->getLocation()
                              : SourceLocation();
}

namespace {
struct CompareTemplateSpecCandidatesForDisplay {
  Sema &S;
  CompareTemplateSpecCandidatesForDisplay(Sema &S) : S(S) {}

  bool operator()(const TemplateSpecCandidate *L,
                  const TemplateSpecCandidate *R) {
    // Fast-path this check.
    if (L == R)
      return false;

    // Assuming that both candidates are not matches...

    // Sort by the ranking of deduction failures.
    if (L->DeductionFailure.Result != R->DeductionFailure.Result)
      return RankDeductionFailure(L->DeductionFailure) <
             RankDeductionFailure(R->DeductionFailure);

    // Sort everything else by location.
    SourceLocation LLoc = GetLocationForCandidate(L);
    SourceLocation RLoc = GetLocationForCandidate(R);

    // Put candidates without locations (e.g. builtins) at the end.
    if (LLoc.isInvalid())
      return false;
    if (RLoc.isInvalid())
      return true;

    return S.SourceMgr.isBeforeInTranslationUnit(LLoc, RLoc);
  }
};
}

/// Diagnose a template argument deduction failure.
/// We are treating these failures as overload failures due to bad
/// deductions.
void TemplateSpecCandidate::NoteDeductionFailure(Sema &S,
                                                 bool ForTakingAddress) {
  DiagnoseBadDeduction(S, FoundDecl, Specialization, // pattern
                       DeductionFailure, /*NumArgs=*/0, ForTakingAddress);
}

void TemplateSpecCandidateSet::destroyCandidates() {
  for (iterator i = begin(), e = end(); i != e; ++i) {
    i->DeductionFailure.Destroy();
  }
}

void TemplateSpecCandidateSet::clear() {
  destroyCandidates();
  Candidates.clear();
}

/// NoteCandidates - When no template specialization match is found, prints
/// diagnostic messages containing the non-matching specializations that form
/// the candidate set.
/// This is analoguous to OverloadCandidateSet::NoteCandidates() with
/// OCD == OCD_AllCandidates and Cand->Viable == false.
void TemplateSpecCandidateSet::NoteCandidates(Sema &S, SourceLocation Loc) {
  // Sort the candidates by position (assuming no candidate is a match).
  // Sorting directly would be prohibitive, so we make a set of pointers
  // and sort those.
  SmallVector<TemplateSpecCandidate *, 32> Cands;
  Cands.reserve(size());
  for (iterator Cand = begin(), LastCand = end(); Cand != LastCand; ++Cand) {
    if (Cand->Specialization)
      Cands.push_back(Cand);
    // Otherwise, this is a non-matching builtin candidate.  We do not,
    // in general, want to list every possible builtin candidate.
  }

  llvm::sort(Cands, CompareTemplateSpecCandidatesForDisplay(S));

  // FIXME: Perhaps rename OverloadsShown and getShowOverloads()
  // for generalization purposes (?).
  const OverloadsShown ShowOverloads = S.Diags.getShowOverloads();

  SmallVectorImpl<TemplateSpecCandidate *>::iterator I, E;
  unsigned CandsShown = 0;
  for (I = Cands.begin(), E = Cands.end(); I != E; ++I) {
    TemplateSpecCandidate *Cand = *I;

    // Set an arbitrary limit on the number of candidates we'll spam
    // the user with.  FIXME: This limit should depend on details of the
    // candidate list.
    if (CandsShown >= 4 && ShowOverloads == Ovl_Best)
      break;
    ++CandsShown;

    assert(Cand->Specialization &&
           "Non-matching built-in candidates are not added to Cands.");
    Cand->NoteDeductionFailure(S, ForTakingAddress);
  }

  if (I != E)
    S.Diag(Loc, diag::note_ovl_too_many_candidates) << int(E - I);
}

// [PossiblyAFunctionType]  -->   [Return]
// NonFunctionType --> NonFunctionType
// R (A) --> R(A)
// R (*)(A) --> R (A)
// R (&)(A) --> R (A)
// R (S::*)(A) --> R (A)
QualType Sema::ExtractUnqualifiedFunctionType(QualType PossiblyAFunctionType) {
  QualType Ret = PossiblyAFunctionType;
  if (const PointerType *ToTypePtr =
    PossiblyAFunctionType->getAs<PointerType>())
    Ret = ToTypePtr->getPointeeType();
  else if (const ReferenceType *ToTypeRef =
    PossiblyAFunctionType->getAs<ReferenceType>())
    Ret = ToTypeRef->getPointeeType();
  else if (const MemberPointerType *MemTypePtr =
    PossiblyAFunctionType->getAs<MemberPointerType>())
    Ret = MemTypePtr->getPointeeType();
  Ret =
    Context.getCanonicalType(Ret).getUnqualifiedType();
  return Ret;
}

static bool completeFunctionType(Sema &S, FunctionDecl *FD, SourceLocation Loc,
                                 bool Complain = true) {
  if (S.getLangOpts().CPlusPlus14 && FD->getReturnType()->isUndeducedType() &&
      S.DeduceReturnType(FD, Loc, Complain))
    return true;

  auto *FPT = FD->getType()->castAs<FunctionProtoType>();
  if (S.getLangOpts().CPlusPlus17 &&
      isUnresolvedExceptionSpec(FPT->getExceptionSpecType()) &&
      !S.ResolveExceptionSpec(Loc, FPT))
    return true;

  return false;
}

namespace {
// A helper class to help with address of function resolution
// - allows us to avoid passing around all those ugly parameters
class AddressOfFunctionResolver {
  Sema& S;
  Expr* SourceExpr;
  const QualType& TargetType;
  QualType TargetFunctionType; // Extracted function type from target type

  bool Complain;
  //DeclAccessPair& ResultFunctionAccessPair;
  ASTContext& Context;

  bool TargetTypeIsNonStaticMemberFunction;
  bool FoundNonTemplateFunction;
  bool StaticMemberFunctionFromBoundPointer;
  bool HasComplained;

  OverloadExpr::FindResult OvlExprInfo;
  OverloadExpr *OvlExpr;
  TemplateArgumentListInfo OvlExplicitTemplateArgs;
  SmallVector<std::pair<DeclAccessPair, FunctionDecl*>, 4> Matches;
  TemplateSpecCandidateSet FailedCandidates;

public:
  AddressOfFunctionResolver(Sema &S, Expr *SourceExpr,
                            const QualType &TargetType, bool Complain)
      : S(S), SourceExpr(SourceExpr), TargetType(TargetType),
        Complain(Complain), Context(S.getASTContext()),
        TargetTypeIsNonStaticMemberFunction(
            !!TargetType->getAs<MemberPointerType>()),
        FoundNonTemplateFunction(false),
        StaticMemberFunctionFromBoundPointer(false),
        HasComplained(false),
        OvlExprInfo(OverloadExpr::find(SourceExpr)),
        OvlExpr(OvlExprInfo.Expression),
        FailedCandidates(OvlExpr->getNameLoc(), /*ForTakingAddress=*/true) {
    ExtractUnqualifiedFunctionTypeFromTargetType();

    if (TargetFunctionType->isFunctionType()) {
      if (UnresolvedMemberExpr *UME = dyn_cast<UnresolvedMemberExpr>(OvlExpr))
        if (!UME->isImplicitAccess() &&
            !S.ResolveSingleFunctionTemplateSpecialization(UME))
          StaticMemberFunctionFromBoundPointer = true;
    } else if (OvlExpr->hasExplicitTemplateArgs()) {
      DeclAccessPair dap;
      if (FunctionDecl *Fn = S.ResolveSingleFunctionTemplateSpecialization(
              OvlExpr, false, &dap)) {
        if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn))
          if (!Method->isStatic()) {
            // If the target type is a non-function type and the function found
            // is a non-static member function, pretend as if that was the
            // target, it's the only possible type to end up with.
            TargetTypeIsNonStaticMemberFunction = true;

            // And skip adding the function if its not in the proper form.
            // We'll diagnose this due to an empty set of functions.
            if (!OvlExprInfo.HasFormOfMemberPointer)
              return;
          }

        Matches.push_back(std::make_pair(dap, Fn));
      }
      return;
    }

    if (OvlExpr->hasExplicitTemplateArgs())
      OvlExpr->copyTemplateArgumentsInto(OvlExplicitTemplateArgs);

    if (FindAllFunctionsThatMatchTargetTypeExactly()) {
      // C++ [over.over]p4:
      //   If more than one function is selected, [...]
      if (Matches.size() > 1 && !eliminiateSuboptimalOverloadCandidates()) {
        if (FoundNonTemplateFunction)
          EliminateAllTemplateMatches();
        else
          EliminateAllExceptMostSpecializedTemplate();
      }
    }

    if (S.getLangOpts().CUDA && Matches.size() > 1)
      EliminateSuboptimalCudaMatches();
  }

  bool hasComplained() const { return HasComplained; }

private:
  bool candidateHasExactlyCorrectType(const FunctionDecl *FD) {
    QualType Discard;
    return Context.hasSameUnqualifiedType(TargetFunctionType, FD->getType()) ||
           S.IsFunctionConversion(FD->getType(), TargetFunctionType, Discard);
  }

  /// \return true if A is considered a better overload candidate for the
  /// desired type than B.
  bool isBetterCandidate(const FunctionDecl *A, const FunctionDecl *B) {
    // If A doesn't have exactly the correct type, we don't want to classify it
    // as "better" than anything else. This way, the user is required to
    // disambiguate for us if there are multiple candidates and no exact match.
    return candidateHasExactlyCorrectType(A) &&
           (!candidateHasExactlyCorrectType(B) ||
            compareEnableIfAttrs(S, A, B) == Comparison::Better);
  }

  /// \return true if we were able to eliminate all but one overload candidate,
  /// false otherwise.
  bool eliminiateSuboptimalOverloadCandidates() {
    // Same algorithm as overload resolution -- one pass to pick the "best",
    // another pass to be sure that nothing is better than the best.
    auto Best = Matches.begin();
    for (auto I = Matches.begin()+1, E = Matches.end(); I != E; ++I)
      if (isBetterCandidate(I->second, Best->second))
        Best = I;

    const FunctionDecl *BestFn = Best->second;
    auto IsBestOrInferiorToBest = [this, BestFn](
        const std::pair<DeclAccessPair, FunctionDecl *> &Pair) {
      return BestFn == Pair.second || isBetterCandidate(BestFn, Pair.second);
    };

    // Note: We explicitly leave Matches unmodified if there isn't a clear best
    // option, so we can potentially give the user a better error
    if (!llvm::all_of(Matches, IsBestOrInferiorToBest))
      return false;
    Matches[0] = *Best;
    Matches.resize(1);
    return true;
  }

  bool isTargetTypeAFunction() const {
    return TargetFunctionType->isFunctionType();
  }

  // [ToType]     [Return]

  // R (*)(A) --> R (A), IsNonStaticMemberFunction = false
  // R (&)(A) --> R (A), IsNonStaticMemberFunction = false
  // R (S::*)(A) --> R (A), IsNonStaticMemberFunction = true
  void inline ExtractUnqualifiedFunctionTypeFromTargetType() {
    TargetFunctionType = S.ExtractUnqualifiedFunctionType(TargetType);
  }

  // return true if any matching specializations were found
  bool AddMatchingTemplateFunction(FunctionTemplateDecl* FunctionTemplate,
                                   const DeclAccessPair& CurAccessFunPair) {
    if (CXXMethodDecl *Method
              = dyn_cast<CXXMethodDecl>(FunctionTemplate->getTemplatedDecl())) {
      // Skip non-static function templates when converting to pointer, and
      // static when converting to member pointer.
      if (Method->isStatic() == TargetTypeIsNonStaticMemberFunction)
        return false;
    }
    else if (TargetTypeIsNonStaticMemberFunction)
      return false;

    // C++ [over.over]p2:
    //   If the name is a function template, template argument deduction is
    //   done (14.8.2.2), and if the argument deduction succeeds, the
    //   resulting template argument list is used to generate a single
    //   function template specialization, which is added to the set of
    //   overloaded functions considered.
    FunctionDecl *Specialization = nullptr;
    TemplateDeductionInfo Info(FailedCandidates.getLocation());
    if (Sema::TemplateDeductionResult Result
          = S.DeduceTemplateArguments(FunctionTemplate,
                                      &OvlExplicitTemplateArgs,
                                      TargetFunctionType, Specialization,
                                      Info, /*IsAddressOfFunction*/true)) {
      // Make a note of the failed deduction for diagnostics.
      FailedCandidates.addCandidate()
          .set(CurAccessFunPair, FunctionTemplate->getTemplatedDecl(),
               MakeDeductionFailureInfo(Context, Result, Info));
      return false;
    }

    // Template argument deduction ensures that we have an exact match or
    // compatible pointer-to-function arguments that would be adjusted by ICS.
    // This function template specicalization works.
    assert(S.isSameOrCompatibleFunctionType(
              Context.getCanonicalType(Specialization->getType()),
              Context.getCanonicalType(TargetFunctionType)));

    if (!S.checkAddressOfFunctionIsAvailable(Specialization))
      return false;

    Matches.push_back(std::make_pair(CurAccessFunPair, Specialization));
    return true;
  }

  bool AddMatchingNonTemplateFunction(NamedDecl* Fn,
                                      const DeclAccessPair& CurAccessFunPair) {
    if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn)) {
      // Skip non-static functions when converting to pointer, and static
      // when converting to member pointer.
      if (Method->isStatic() == TargetTypeIsNonStaticMemberFunction)
        return false;
    }
    else if (TargetTypeIsNonStaticMemberFunction)
      return false;

    if (FunctionDecl *FunDecl = dyn_cast<FunctionDecl>(Fn)) {
      if (S.getLangOpts().CUDA)
        if (FunctionDecl *Caller = S.getCurFunctionDecl(/*AllowLambda=*/true))
          if (!Caller->isImplicit() && !S.IsAllowedCUDACall(Caller, FunDecl))
            return false;
      if (FunDecl->isMultiVersion()) {
        const auto *TA = FunDecl->getAttr<TargetAttr>();
        if (TA && !TA->isDefaultVersion())
          return false;
        const auto *TVA = FunDecl->getAttr<TargetVersionAttr>();
        if (TVA && !TVA->isDefaultVersion())
          return false;
      }

      // If any candidate has a placeholder return type, trigger its deduction
      // now.
      if (completeFunctionType(S, FunDecl, SourceExpr->getBeginLoc(),
                               Complain)) {
        HasComplained |= Complain;
        return false;
      }

      if (!S.checkAddressOfFunctionIsAvailable(FunDecl))
        return false;

      // If we're in C, we need to support types that aren't exactly identical.
      if (!S.getLangOpts().CPlusPlus ||
          candidateHasExactlyCorrectType(FunDecl)) {
        Matches.push_back(std::make_pair(
            CurAccessFunPair, cast<FunctionDecl>(FunDecl->getCanonicalDecl())));
        FoundNonTemplateFunction = true;
        return true;
      }
    }

    return false;
  }

  bool FindAllFunctionsThatMatchTargetTypeExactly() {
    bool Ret = false;

    // If the overload expression doesn't have the form of a pointer to
    // member, don't try to convert it to a pointer-to-member type.
    if (IsInvalidFormOfPointerToMemberFunction())
      return false;

    for (UnresolvedSetIterator I = OvlExpr->decls_begin(),
                               E = OvlExpr->decls_end();
         I != E; ++I) {
      // Look through any using declarations to find the underlying function.
      NamedDecl *Fn = (*I)->getUnderlyingDecl();

      // C++ [over.over]p3:
      //   Non-member functions and static member functions match
      //   targets of type "pointer-to-function" or "reference-to-function."
      //   Nonstatic member functions match targets of
      //   type "pointer-to-member-function."
      // Note that according to DR 247, the containing class does not matter.
      if (FunctionTemplateDecl *FunctionTemplate
                                        = dyn_cast<FunctionTemplateDecl>(Fn)) {
        if (AddMatchingTemplateFunction(FunctionTemplate, I.getPair()))
          Ret = true;
      }
      // If we have explicit template arguments supplied, skip non-templates.
      else if (!OvlExpr->hasExplicitTemplateArgs() &&
               AddMatchingNonTemplateFunction(Fn, I.getPair()))
        Ret = true;
    }
    assert(Ret || Matches.empty());
    return Ret;
  }

  void EliminateAllExceptMostSpecializedTemplate() {
    //   [...] and any given function template specialization F1 is
    //   eliminated if the set contains a second function template
    //   specialization whose function template is more specialized
    //   than the function template of F1 according to the partial
    //   ordering rules of 14.5.5.2.

    // The algorithm specified above is quadratic. We instead use a
    // two-pass algorithm (similar to the one used to identify the
    // best viable function in an overload set) that identifies the
    // best function template (if it exists).

    UnresolvedSet<4> MatchesCopy; // TODO: avoid!
    for (unsigned I = 0, E = Matches.size(); I != E; ++I)
      MatchesCopy.addDecl(Matches[I].second, Matches[I].first.getAccess());

    // TODO: It looks like FailedCandidates does not serve much purpose
    // here, since the no_viable diagnostic has index 0.
    UnresolvedSetIterator Result = S.getMostSpecialized(
        MatchesCopy.begin(), MatchesCopy.end(), FailedCandidates,
        SourceExpr->getBeginLoc(), S.PDiag(),
        S.PDiag(diag::err_addr_ovl_ambiguous)
            << Matches[0].second->getDeclName(),
        S.PDiag(diag::note_ovl_candidate)
            << (unsigned)oc_function << (unsigned)ocs_described_template,
        Complain, TargetFunctionType);

    if (Result != MatchesCopy.end()) {
      // Make it the first and only element
      Matches[0].first = Matches[Result - MatchesCopy.begin()].first;
      Matches[0].second = cast<FunctionDecl>(*Result);
      Matches.resize(1);
    } else
      HasComplained |= Complain;
  }

  void EliminateAllTemplateMatches() {
    //   [...] any function template specializations in the set are
    //   eliminated if the set also contains a non-template function, [...]
    for (unsigned I = 0, N = Matches.size(); I != N; ) {
      if (Matches[I].second->getPrimaryTemplate() == nullptr)
        ++I;
      else {
        Matches[I] = Matches[--N];
        Matches.resize(N);
      }
    }
  }

  void EliminateSuboptimalCudaMatches() {
    S.EraseUnwantedCUDAMatches(S.getCurFunctionDecl(/*AllowLambda=*/true),
                               Matches);
  }

public:
  void ComplainNoMatchesFound() const {
    assert(Matches.empty());
    S.Diag(OvlExpr->getBeginLoc(), diag::err_addr_ovl_no_viable)
        << OvlExpr->getName() << TargetFunctionType
        << OvlExpr->getSourceRange();
    if (FailedCandidates.empty())
      S.NoteAllOverloadCandidates(OvlExpr, TargetFunctionType,
                                  /*TakingAddress=*/true);
    else {
      // We have some deduction failure messages. Use them to diagnose
      // the function templates, and diagnose the non-template candidates
      // normally.
      for (UnresolvedSetIterator I = OvlExpr->decls_begin(),
                                 IEnd = OvlExpr->decls_end();
           I != IEnd; ++I)
        if (FunctionDecl *Fun =
                dyn_cast<FunctionDecl>((*I)->getUnderlyingDecl()))
          if (!functionHasPassObjectSizeParams(Fun))
            S.NoteOverloadCandidate(*I, Fun, CRK_None, TargetFunctionType,
                                    /*TakingAddress=*/true);
      FailedCandidates.NoteCandidates(S, OvlExpr->getBeginLoc());
    }
  }

  bool IsInvalidFormOfPointerToMemberFunction() const {
    return TargetTypeIsNonStaticMemberFunction &&
      !OvlExprInfo.HasFormOfMemberPointer;
  }

  void ComplainIsInvalidFormOfPointerToMemberFunction() const {
      // TODO: Should we condition this on whether any functions might
      // have matched, or is it more appropriate to do that in callers?
      // TODO: a fixit wouldn't hurt.
      S.Diag(OvlExpr->getNameLoc(), diag::err_addr_ovl_no_qualifier)
        << TargetType << OvlExpr->getSourceRange();
  }

  bool IsStaticMemberFunctionFromBoundPointer() const {
    return StaticMemberFunctionFromBoundPointer;
  }

  void ComplainIsStaticMemberFunctionFromBoundPointer() const {
    S.Diag(OvlExpr->getBeginLoc(),
           diag::err_invalid_form_pointer_member_function)
        << OvlExpr->getSourceRange();
  }

  void ComplainOfInvalidConversion() const {
    S.Diag(OvlExpr->getBeginLoc(), diag::err_addr_ovl_not_func_ptrref)
        << OvlExpr->getName() << TargetType;
  }

  void ComplainMultipleMatchesFound() const {
    assert(Matches.size() > 1);
    S.Diag(OvlExpr->getBeginLoc(), diag::err_addr_ovl_ambiguous)
        << OvlExpr->getName() << OvlExpr->getSourceRange();
    S.NoteAllOverloadCandidates(OvlExpr, TargetFunctionType,
                                /*TakingAddress=*/true);
  }

  bool hadMultipleCandidates() const { return (OvlExpr->getNumDecls() > 1); }

  int getNumMatches() const { return Matches.size(); }

  FunctionDecl* getMatchingFunctionDecl() const {
    if (Matches.size() != 1) return nullptr;
    return Matches[0].second;
  }

  const DeclAccessPair* getMatchingFunctionAccessPair() const {
    if (Matches.size() != 1) return nullptr;
    return &Matches[0].first;
  }
};
}

/// ResolveAddressOfOverloadedFunction - Try to resolve the address of
/// an overloaded function (C++ [over.over]), where @p From is an
/// expression with overloaded function type and @p ToType is the type
/// we're trying to resolve to. For example:
///
/// @code
/// int f(double);
/// int f(int);
///
/// int (*pfd)(double) = f; // selects f(double)
/// @endcode
///
/// This routine returns the resulting FunctionDecl if it could be
/// resolved, and NULL otherwise. When @p Complain is true, this
/// routine will emit diagnostics if there is an error.
FunctionDecl *
Sema::ResolveAddressOfOverloadedFunction(Expr *AddressOfExpr,
                                         QualType TargetType,
                                         bool Complain,
                                         DeclAccessPair &FoundResult,
                                         bool *pHadMultipleCandidates) {
  assert(AddressOfExpr->getType() == Context.OverloadTy);

  AddressOfFunctionResolver Resolver(*this, AddressOfExpr, TargetType,
                                     Complain);
  int NumMatches = Resolver.getNumMatches();
  FunctionDecl *Fn = nullptr;
  bool ShouldComplain = Complain && !Resolver.hasComplained();
  if (NumMatches == 0 && ShouldComplain) {
    if (Resolver.IsInvalidFormOfPointerToMemberFunction())
      Resolver.ComplainIsInvalidFormOfPointerToMemberFunction();
    else
      Resolver.ComplainNoMatchesFound();
  }
  else if (NumMatches > 1 && ShouldComplain)
    Resolver.ComplainMultipleMatchesFound();
  else if (NumMatches == 1) {
    Fn = Resolver.getMatchingFunctionDecl();
    assert(Fn);
    if (auto *FPT = Fn->getType()->getAs<FunctionProtoType>())
      ResolveExceptionSpec(AddressOfExpr->getExprLoc(), FPT);
    FoundResult = *Resolver.getMatchingFunctionAccessPair();
    if (Complain) {
      if (Resolver.IsStaticMemberFunctionFromBoundPointer())
        Resolver.ComplainIsStaticMemberFunctionFromBoundPointer();
      else
        CheckAddressOfMemberAccess(AddressOfExpr, FoundResult);
    }
  }

  if (pHadMultipleCandidates)
    *pHadMultipleCandidates = Resolver.hadMultipleCandidates();
  return Fn;
}

/// Given an expression that refers to an overloaded function, try to
/// resolve that function to a single function that can have its address taken.
/// This will modify `Pair` iff it returns non-null.
///
/// This routine can only succeed if from all of the candidates in the overload
/// set for SrcExpr that can have their addresses taken, there is one candidate
/// that is more constrained than the rest.
FunctionDecl *
Sema::resolveAddressOfSingleOverloadCandidate(Expr *E, DeclAccessPair &Pair) {
  OverloadExpr::FindResult R = OverloadExpr::find(E);
  OverloadExpr *Ovl = R.Expression;
  bool IsResultAmbiguous = false;
  FunctionDecl *Result = nullptr;
  DeclAccessPair DAP;
  SmallVector<FunctionDecl *, 2> AmbiguousDecls;

  auto CheckMoreConstrained = [&](FunctionDecl *FD1,
                                  FunctionDecl *FD2) -> std::optional<bool> {
    if (FunctionDecl *MF = FD1->getInstantiatedFromMemberFunction())
      FD1 = MF;
    if (FunctionDecl *MF = FD2->getInstantiatedFromMemberFunction())
      FD2 = MF;
    SmallVector<const Expr *, 1> AC1, AC2;
    FD1->getAssociatedConstraints(AC1);
    FD2->getAssociatedConstraints(AC2);
    bool AtLeastAsConstrained1, AtLeastAsConstrained2;
    if (IsAtLeastAsConstrained(FD1, AC1, FD2, AC2, AtLeastAsConstrained1))
      return std::nullopt;
    if (IsAtLeastAsConstrained(FD2, AC2, FD1, AC1, AtLeastAsConstrained2))
      return std::nullopt;
    if (AtLeastAsConstrained1 == AtLeastAsConstrained2)
      return std::nullopt;
    return AtLeastAsConstrained1;
  };

  // Don't use the AddressOfResolver because we're specifically looking for
  // cases where we have one overload candidate that lacks
  // enable_if/pass_object_size/...
  for (auto I = Ovl->decls_begin(), E = Ovl->decls_end(); I != E; ++I) {
    auto *FD = dyn_cast<FunctionDecl>(I->getUnderlyingDecl());
    if (!FD)
      return nullptr;

    if (!checkAddressOfFunctionIsAvailable(FD))
      continue;

    // We have more than one result - see if it is more constrained than the
    // previous one.
    if (Result) {
      std::optional<bool> MoreConstrainedThanPrevious =
          CheckMoreConstrained(FD, Result);
      if (!MoreConstrainedThanPrevious) {
        IsResultAmbiguous = true;
        AmbiguousDecls.push_back(FD);
        continue;
      }
      if (!*MoreConstrainedThanPrevious)
        continue;
      // FD is more constrained - replace Result with it.
    }
    IsResultAmbiguous = false;
    DAP = I.getPair();
    Result = FD;
  }

  if (IsResultAmbiguous)
    return nullptr;

  if (Result) {
    SmallVector<const Expr *, 1> ResultAC;
    // We skipped over some ambiguous declarations which might be ambiguous with
    // the selected result.
    for (FunctionDecl *Skipped : AmbiguousDecls)
      if (!CheckMoreConstrained(Skipped, Result))
        return nullptr;
    Pair = DAP;
  }
  return Result;
}

/// Given an overloaded function, tries to turn it into a non-overloaded
/// function reference using resolveAddressOfSingleOverloadCandidate. This
/// will perform access checks, diagnose the use of the resultant decl, and, if
/// requested, potentially perform a function-to-pointer decay.
///
/// Returns false if resolveAddressOfSingleOverloadCandidate fails.
/// Otherwise, returns true. This may emit diagnostics and return true.
bool Sema::resolveAndFixAddressOfSingleOverloadCandidate(
    ExprResult &SrcExpr, bool DoFunctionPointerConversion) {
  Expr *E = SrcExpr.get();
  assert(E->getType() == Context.OverloadTy && "SrcExpr must be an overload");

  DeclAccessPair DAP;
  FunctionDecl *Found = resolveAddressOfSingleOverloadCandidate(E, DAP);
  if (!Found || Found->isCPUDispatchMultiVersion() ||
      Found->isCPUSpecificMultiVersion())
    return false;

  // Emitting multiple diagnostics for a function that is both inaccessible and
  // unavailable is consistent with our behavior elsewhere. So, always check
  // for both.
  DiagnoseUseOfDecl(Found, E->getExprLoc());
  CheckAddressOfMemberAccess(E, DAP);
  Expr *Fixed = FixOverloadedFunctionReference(E, DAP, Found);
  if (DoFunctionPointerConversion && Fixed->getType()->isFunctionType())
    SrcExpr = DefaultFunctionArrayConversion(Fixed, /*Diagnose=*/false);
  else
    SrcExpr = Fixed;
  return true;
}

/// Given an expression that refers to an overloaded function, try to
/// resolve that overloaded function expression down to a single function.
///
/// This routine can only resolve template-ids that refer to a single function
/// template, where that template-id refers to a single template whose template
/// arguments are either provided by the template-id or have defaults,
/// as described in C++0x [temp.arg.explicit]p3.
///
/// If no template-ids are found, no diagnostics are emitted and NULL is
/// returned.
FunctionDecl *
Sema::ResolveSingleFunctionTemplateSpecialization(OverloadExpr *ovl,
                                                  bool Complain,
                                                  DeclAccessPair *FoundResult) {
  // C++ [over.over]p1:
  //   [...] [Note: any redundant set of parentheses surrounding the
  //   overloaded function name is ignored (5.1). ]
  // C++ [over.over]p1:
  //   [...] The overloaded function name can be preceded by the &
  //   operator.

  // If we didn't actually find any template-ids, we're done.
  if (!ovl->hasExplicitTemplateArgs())
    return nullptr;

  TemplateArgumentListInfo ExplicitTemplateArgs;
  ovl->copyTemplateArgumentsInto(ExplicitTemplateArgs);
  TemplateSpecCandidateSet FailedCandidates(ovl->getNameLoc());

  // Look through all of the overloaded functions, searching for one
  // whose type matches exactly.
  FunctionDecl *Matched = nullptr;
  for (UnresolvedSetIterator I = ovl->decls_begin(),
         E = ovl->decls_end(); I != E; ++I) {
    // C++0x [temp.arg.explicit]p3:
    //   [...] In contexts where deduction is done and fails, or in contexts
    //   where deduction is not done, if a template argument list is
    //   specified and it, along with any default template arguments,
    //   identifies a single function template specialization, then the
    //   template-id is an lvalue for the function template specialization.
    FunctionTemplateDecl *FunctionTemplate
      = cast<FunctionTemplateDecl>((*I)->getUnderlyingDecl());

    // C++ [over.over]p2:
    //   If the name is a function template, template argument deduction is
    //   done (14.8.2.2), and if the argument deduction succeeds, the
    //   resulting template argument list is used to generate a single
    //   function template specialization, which is added to the set of
    //   overloaded functions considered.
    FunctionDecl *Specialization = nullptr;
    TemplateDeductionInfo Info(FailedCandidates.getLocation());
    if (TemplateDeductionResult Result
          = DeduceTemplateArguments(FunctionTemplate, &ExplicitTemplateArgs,
                                    Specialization, Info,
                                    /*IsAddressOfFunction*/true)) {
      // Make a note of the failed deduction for diagnostics.
      // TODO: Actually use the failed-deduction info?
      FailedCandidates.addCandidate()
          .set(I.getPair(), FunctionTemplate->getTemplatedDecl(),
               MakeDeductionFailureInfo(Context, Result, Info));
      continue;
    }

    assert(Specialization && "no specialization and no error?");

    // Multiple matches; we can't resolve to a single declaration.
    if (Matched) {
      if (Complain) {
        Diag(ovl->getExprLoc(), diag::err_addr_ovl_ambiguous)
          << ovl->getName();
        NoteAllOverloadCandidates(ovl);
      }
      return nullptr;
    }

    Matched = Specialization;
    if (FoundResult) *FoundResult = I.getPair();
  }

  if (Matched &&
      completeFunctionType(*this, Matched, ovl->getExprLoc(), Complain))
    return nullptr;

  return Matched;
}

// Resolve and fix an overloaded expression that can be resolved
// because it identifies a single function template specialization.
//
// Last three arguments should only be supplied if Complain = true
//
// Return true if it was logically possible to so resolve the
// expression, regardless of whether or not it succeeded.  Always
// returns true if 'complain' is set.
bool Sema::ResolveAndFixSingleFunctionTemplateSpecialization(
    ExprResult &SrcExpr, bool doFunctionPointerConversion, bool complain,
    SourceRange OpRangeForComplaining, QualType DestTypeForComplaining,
    unsigned DiagIDForComplaining) {
  assert(SrcExpr.get()->getType() == Context.OverloadTy);

  OverloadExpr::FindResult ovl = OverloadExpr::find(SrcExpr.get());

  DeclAccessPair found;
  ExprResult SingleFunctionExpression;
  if (FunctionDecl *fn = ResolveSingleFunctionTemplateSpecialization(
                           ovl.Expression, /*complain*/ false, &found)) {
    if (DiagnoseUseOfDecl(fn, SrcExpr.get()->getBeginLoc())) {
      SrcExpr = ExprError();
      return true;
    }

    // It is only correct to resolve to an instance method if we're
    // resolving a form that's permitted to be a pointer to member.
    // Otherwise we'll end up making a bound member expression, which
    // is illegal in all the contexts we resolve like this.
    if (!ovl.HasFormOfMemberPointer &&
        isa<CXXMethodDecl>(fn) &&
        cast<CXXMethodDecl>(fn)->isInstance()) {
      if (!complain) return false;

      Diag(ovl.Expression->getExprLoc(),
           diag::err_bound_member_function)
        << 0 << ovl.Expression->getSourceRange();

      // TODO: I believe we only end up here if there's a mix of
      // static and non-static candidates (otherwise the expression
      // would have 'bound member' type, not 'overload' type).
      // Ideally we would note which candidate was chosen and why
      // the static candidates were rejected.
      SrcExpr = ExprError();
      return true;
    }

    // Fix the expression to refer to 'fn'.
    SingleFunctionExpression =
        FixOverloadedFunctionReference(SrcExpr.get(), found, fn);

    // If desired, do function-to-pointer decay.
    if (doFunctionPointerConversion) {
      SingleFunctionExpression =
        DefaultFunctionArrayLvalueConversion(SingleFunctionExpression.get());
      if (SingleFunctionExpression.isInvalid()) {
        SrcExpr = ExprError();
        return true;
      }
    }
  }

  if (!SingleFunctionExpression.isUsable()) {
    if (complain) {
      Diag(OpRangeForComplaining.getBegin(), DiagIDForComplaining)
        << ovl.Expression->getName()
        << DestTypeForComplaining
        << OpRangeForComplaining
        << ovl.Expression->getQualifierLoc().getSourceRange();
      NoteAllOverloadCandidates(SrcExpr.get());

      SrcExpr = ExprError();
      return true;
    }

    return false;
  }

  SrcExpr = SingleFunctionExpression;
  return true;
}

/// Add a single candidate to the overload set.
static void AddOverloadedCallCandidate(Sema &S,
                                       DeclAccessPair FoundDecl,
                                 TemplateArgumentListInfo *ExplicitTemplateArgs,
                                       ArrayRef<Expr *> Args,
                                       OverloadCandidateSet &CandidateSet,
                                       bool PartialOverloading,
                                       bool KnownValid) {
  NamedDecl *Callee = FoundDecl.getDecl();
  if (isa<UsingShadowDecl>(Callee))
    Callee = cast<UsingShadowDecl>(Callee)->getTargetDecl();

  if (FunctionDecl *Func = dyn_cast<FunctionDecl>(Callee)) {
    if (ExplicitTemplateArgs) {
      assert(!KnownValid && "Explicit template arguments?");
      return;
    }
    // Prevent ill-formed function decls to be added as overload candidates.
    if (!isa<FunctionProtoType>(Func->getType()->getAs<FunctionType>()))
      return;

    S.AddOverloadCandidate(Func, FoundDecl, Args, CandidateSet,
                           /*SuppressUserConversions=*/false,
                           PartialOverloading);
    return;
  }

  if (FunctionTemplateDecl *FuncTemplate
      = dyn_cast<FunctionTemplateDecl>(Callee)) {
    S.AddTemplateOverloadCandidate(FuncTemplate, FoundDecl,
                                   ExplicitTemplateArgs, Args, CandidateSet,
                                   /*SuppressUserConversions=*/false,
                                   PartialOverloading);
    return;
  }

  assert(!KnownValid && "unhandled case in overloaded call candidate");
}

/// Add the overload candidates named by callee and/or found by argument
/// dependent lookup to the given overload set.
void Sema::AddOverloadedCallCandidates(UnresolvedLookupExpr *ULE,
                                       ArrayRef<Expr *> Args,
                                       OverloadCandidateSet &CandidateSet,
                                       bool PartialOverloading) {

#ifndef NDEBUG
  // Verify that ArgumentDependentLookup is consistent with the rules
  // in C++0x [basic.lookup.argdep]p3:
  //
  //   Let X be the lookup set produced by unqualified lookup (3.4.1)
  //   and let Y be the lookup set produced by argument dependent
  //   lookup (defined as follows). If X contains
  //
  //     -- a declaration of a class member, or
  //
  //     -- a block-scope function declaration that is not a
  //        using-declaration, or
  //
  //     -- a declaration that is neither a function or a function
  //        template
  //
  //   then Y is empty.

  if (ULE->requiresADL()) {
    for (UnresolvedLookupExpr::decls_iterator I = ULE->decls_begin(),
           E = ULE->decls_end(); I != E; ++I) {
      assert(!(*I)->getDeclContext()->isRecord());
      assert(isa<UsingShadowDecl>(*I) ||
             !(*I)->getDeclContext()->isFunctionOrMethod());
      assert((*I)->getUnderlyingDecl()->isFunctionOrFunctionTemplate());
    }
  }
#endif

  // It would be nice to avoid this copy.
  TemplateArgumentListInfo TABuffer;
  TemplateArgumentListInfo *ExplicitTemplateArgs = nullptr;
  if (ULE->hasExplicitTemplateArgs()) {
    ULE->copyTemplateArgumentsInto(TABuffer);
    ExplicitTemplateArgs = &TABuffer;
  }

  for (UnresolvedLookupExpr::decls_iterator I = ULE->decls_begin(),
         E = ULE->decls_end(); I != E; ++I)
    AddOverloadedCallCandidate(*this, I.getPair(), ExplicitTemplateArgs, Args,
                               CandidateSet, PartialOverloading,
                               /*KnownValid*/ true);

  if (ULE->requiresADL())
    AddArgumentDependentLookupCandidates(ULE->getName(), ULE->getExprLoc(),
                                         Args, ExplicitTemplateArgs,
                                         CandidateSet, PartialOverloading);
}

/// Add the call candidates from the given set of lookup results to the given
/// overload set. Non-function lookup results are ignored.
void Sema::AddOverloadedCallCandidates(
    LookupResult &R, TemplateArgumentListInfo *ExplicitTemplateArgs,
    ArrayRef<Expr *> Args, OverloadCandidateSet &CandidateSet) {
  for (LookupResult::iterator I = R.begin(), E = R.end(); I != E; ++I)
    AddOverloadedCallCandidate(*this, I.getPair(), ExplicitTemplateArgs, Args,
                               CandidateSet, false, /*KnownValid*/ false);
}

/// Determine whether a declaration with the specified name could be moved into
/// a different namespace.
static bool canBeDeclaredInNamespace(const DeclarationName &Name) {
  switch (Name.getCXXOverloadedOperator()) {
  case OO_New: case OO_Array_New:
  case OO_Delete: case OO_Array_Delete:
    return false;

  default:
    return true;
  }
}

/// Attempt to recover from an ill-formed use of a non-dependent name in a
/// template, where the non-dependent name was declared after the template
/// was defined. This is common in code written for a compilers which do not
/// correctly implement two-stage name lookup.
///
/// Returns true if a viable candidate was found and a diagnostic was issued.
static bool DiagnoseTwoPhaseLookup(
    Sema &SemaRef, SourceLocation FnLoc, const CXXScopeSpec &SS,
    LookupResult &R, OverloadCandidateSet::CandidateSetKind CSK,
    TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
    CXXRecordDecl **FoundInClass = nullptr) {
  if (!SemaRef.inTemplateInstantiation() || !SS.isEmpty())
    return false;

  for (DeclContext *DC = SemaRef.CurContext; DC; DC = DC->getParent()) {
    if (DC->isTransparentContext())
      continue;

    SemaRef.LookupQualifiedName(R, DC);

    if (!R.empty()) {
      R.suppressDiagnostics();

      OverloadCandidateSet Candidates(FnLoc, CSK);
      SemaRef.AddOverloadedCallCandidates(R, ExplicitTemplateArgs, Args,
                                          Candidates);

      OverloadCandidateSet::iterator Best;
      OverloadingResult OR =
          Candidates.BestViableFunction(SemaRef, FnLoc, Best);

      if (auto *RD = dyn_cast<CXXRecordDecl>(DC)) {
        // We either found non-function declarations or a best viable function
        // at class scope. A class-scope lookup result disables ADL. Don't
        // look past this, but let the caller know that we found something that
        // either is, or might be, usable in this class.
        if (FoundInClass) {
          *FoundInClass = RD;
          if (OR == OR_Success) {
            R.clear();
            R.addDecl(Best->FoundDecl.getDecl(), Best->FoundDecl.getAccess());
            R.resolveKind();
          }
        }
        return false;
      }

      if (OR != OR_Success) {
        // There wasn't a unique best function or function template.
        return false;
      }

      // Find the namespaces where ADL would have looked, and suggest
      // declaring the function there instead.
      Sema::AssociatedNamespaceSet AssociatedNamespaces;
      Sema::AssociatedClassSet AssociatedClasses;
      SemaRef.FindAssociatedClassesAndNamespaces(FnLoc, Args,
                                                 AssociatedNamespaces,
                                                 AssociatedClasses);
      Sema::AssociatedNamespaceSet SuggestedNamespaces;
      if (canBeDeclaredInNamespace(R.getLookupName())) {
        DeclContext *Std = SemaRef.getStdNamespace();
        for (Sema::AssociatedNamespaceSet::iterator
               it = AssociatedNamespaces.begin(),
               end = AssociatedNamespaces.end(); it != end; ++it) {
          // Never suggest declaring a function within namespace 'std'.
          if (Std && Std->Encloses(*it))
            continue;

          // Never suggest declaring a function within a namespace with a
          // reserved name, like __gnu_cxx.
          NamespaceDecl *NS = dyn_cast<NamespaceDecl>(*it);
          if (NS &&
              NS->getQualifiedNameAsString().find("__") != std::string::npos)
            continue;

          SuggestedNamespaces.insert(*it);
        }
      }

      SemaRef.Diag(R.getNameLoc(), diag::err_not_found_by_two_phase_lookup)
        << R.getLookupName();
      if (SuggestedNamespaces.empty()) {
        SemaRef.Diag(Best->Function->getLocation(),
                     diag::note_not_found_by_two_phase_lookup)
          << R.getLookupName() << 0;
      } else if (SuggestedNamespaces.size() == 1) {
        SemaRef.Diag(Best->Function->getLocation(),
                     diag::note_not_found_by_two_phase_lookup)
          << R.getLookupName() << 1 << *SuggestedNamespaces.begin();
      } else {
        // FIXME: It would be useful to list the associated namespaces here,
        // but the diagnostics infrastructure doesn't provide a way to produce
        // a localized representation of a list of items.
        SemaRef.Diag(Best->Function->getLocation(),
                     diag::note_not_found_by_two_phase_lookup)
          << R.getLookupName() << 2;
      }

      // Try to recover by calling this function.
      return true;
    }

    R.clear();
  }

  return false;
}

/// Attempt to recover from ill-formed use of a non-dependent operator in a
/// template, where the non-dependent operator was declared after the template
/// was defined.
///
/// Returns true if a viable candidate was found and a diagnostic was issued.
static bool
DiagnoseTwoPhaseOperatorLookup(Sema &SemaRef, OverloadedOperatorKind Op,
                               SourceLocation OpLoc,
                               ArrayRef<Expr *> Args) {
  DeclarationName OpName =
    SemaRef.Context.DeclarationNames.getCXXOperatorName(Op);
  LookupResult R(SemaRef, OpName, OpLoc, Sema::LookupOperatorName);
  return DiagnoseTwoPhaseLookup(SemaRef, OpLoc, CXXScopeSpec(), R,
                                OverloadCandidateSet::CSK_Operator,
                                /*ExplicitTemplateArgs=*/nullptr, Args);
}

namespace {
class BuildRecoveryCallExprRAII {
  Sema &SemaRef;
  Sema::SatisfactionStackResetRAII SatStack;

public:
  BuildRecoveryCallExprRAII(Sema &S) : SemaRef(S), SatStack(S) {
    assert(SemaRef.IsBuildingRecoveryCallExpr == false);
    SemaRef.IsBuildingRecoveryCallExpr = true;
  }

  ~BuildRecoveryCallExprRAII() { SemaRef.IsBuildingRecoveryCallExpr = false; }
};
}

/// Attempts to recover from a call where no functions were found.
///
/// This function will do one of three things:
///  * Diagnose, recover, and return a recovery expression.
///  * Diagnose, fail to recover, and return ExprError().
///  * Do not diagnose, do not recover, and return ExprResult(). The caller is
///    expected to diagnose as appropriate.
static ExprResult
BuildRecoveryCallExpr(Sema &SemaRef, Scope *S, Expr *Fn,
                      UnresolvedLookupExpr *ULE,
                      SourceLocation LParenLoc,
                      MutableArrayRef<Expr *> Args,
                      SourceLocation RParenLoc,
                      bool EmptyLookup, bool AllowTypoCorrection) {
  // Do not try to recover if it is already building a recovery call.
  // This stops infinite loops for template instantiations like
  //
  // template <typename T> auto foo(T t) -> decltype(foo(t)) {}
  // template <typename T> auto foo(T t) -> decltype(foo(&t)) {}
  if (SemaRef.IsBuildingRecoveryCallExpr)
    return ExprResult();
  BuildRecoveryCallExprRAII RCE(SemaRef);

  CXXScopeSpec SS;
  SS.Adopt(ULE->getQualifierLoc());
  SourceLocation TemplateKWLoc = ULE->getTemplateKeywordLoc();

  TemplateArgumentListInfo TABuffer;
  TemplateArgumentListInfo *ExplicitTemplateArgs = nullptr;
  if (ULE->hasExplicitTemplateArgs()) {
    ULE->copyTemplateArgumentsInto(TABuffer);
    ExplicitTemplateArgs = &TABuffer;
  }

  LookupResult R(SemaRef, ULE->getName(), ULE->getNameLoc(),
                 Sema::LookupOrdinaryName);
  CXXRecordDecl *FoundInClass = nullptr;
  if (DiagnoseTwoPhaseLookup(SemaRef, Fn->getExprLoc(), SS, R,
                             OverloadCandidateSet::CSK_Normal,
                             ExplicitTemplateArgs, Args, &FoundInClass)) {
    // OK, diagnosed a two-phase lookup issue.
  } else if (EmptyLookup) {
    // Try to recover from an empty lookup with typo correction.
    R.clear();
    NoTypoCorrectionCCC NoTypoValidator{};
    FunctionCallFilterCCC FunctionCallValidator(SemaRef, Args.size(),
                                                ExplicitTemplateArgs != nullptr,
                                                dyn_cast<MemberExpr>(Fn));
    CorrectionCandidateCallback &Validator =
        AllowTypoCorrection
            ? static_cast<CorrectionCandidateCallback &>(FunctionCallValidator)
            : static_cast<CorrectionCandidateCallback &>(NoTypoValidator);
    if (SemaRef.DiagnoseEmptyLookup(S, SS, R, Validator, ExplicitTemplateArgs,
                                    Args))
      return ExprError();
  } else if (FoundInClass && SemaRef.getLangOpts().MSVCCompat) {
    // We found a usable declaration of the name in a dependent base of some
    // enclosing class.
    // FIXME: We should also explain why the candidates found by name lookup
    // were not viable.
    if (SemaRef.DiagnoseDependentMemberLookup(R))
      return ExprError();
  } else {
    // We had viable candidates and couldn't recover; let the caller diagnose
    // this.
    return ExprResult();
  }

  // If we get here, we should have issued a diagnostic and formed a recovery
  // lookup result.
  assert(!R.empty() && "lookup results empty despite recovery");

  // If recovery created an ambiguity, just bail out.
  if (R.isAmbiguous()) {
    R.suppressDiagnostics();
    return ExprError();
  }

  // Build an implicit member call if appropriate.  Just drop the
  // casts and such from the call, we don't really care.
  ExprResult NewFn = ExprError();
  if ((*R.begin())->isCXXClassMember())
    NewFn = SemaRef.BuildPossibleImplicitMemberExpr(SS, TemplateKWLoc, R,
                                                    ExplicitTemplateArgs, S);
  else if (ExplicitTemplateArgs || TemplateKWLoc.isValid())
    NewFn = SemaRef.BuildTemplateIdExpr(SS, TemplateKWLoc, R, false,
                                        ExplicitTemplateArgs);
  else
    NewFn = SemaRef.BuildDeclarationNameExpr(SS, R, false);

  if (NewFn.isInvalid())
    return ExprError();

  // This shouldn't cause an infinite loop because we're giving it
  // an expression with viable lookup results, which should never
  // end up here.
  return SemaRef.BuildCallExpr(/*Scope*/ nullptr, NewFn.get(), LParenLoc,
                               MultiExprArg(Args.data(), Args.size()),
                               RParenLoc);
}

/// Constructs and populates an OverloadedCandidateSet from
/// the given function.
/// \returns true when an the ExprResult output parameter has been set.
bool Sema::buildOverloadedCallSet(Scope *S, Expr *Fn,
                                  UnresolvedLookupExpr *ULE,
                                  MultiExprArg Args,
                                  SourceLocation RParenLoc,
                                  OverloadCandidateSet *CandidateSet,
                                  ExprResult *Result) {
#ifndef NDEBUG
  if (ULE->requiresADL()) {
    // To do ADL, we must have found an unqualified name.
    assert(!ULE->getQualifier() && "qualified name with ADL");

    // We don't perform ADL for implicit declarations of builtins.
    // Verify that this was correctly set up.
    FunctionDecl *F;
    if (ULE->decls_begin() != ULE->decls_end() &&
        ULE->decls_begin() + 1 == ULE->decls_end() &&
        (F = dyn_cast<FunctionDecl>(*ULE->decls_begin())) &&
        F->getBuiltinID() && F->isImplicit())
      llvm_unreachable("performing ADL for builtin");

    // We don't perform ADL in C.
    assert(getLangOpts().CPlusPlus && "ADL enabled in C");
  }
#endif

  UnbridgedCastsSet UnbridgedCasts;
  if (checkArgPlaceholdersForOverload(*this, Args, UnbridgedCasts)) {
    *Result = ExprError();
    return true;
  }

  // Add the functions denoted by the callee to the set of candidate
  // functions, including those from argument-dependent lookup.
  AddOverloadedCallCandidates(ULE, Args, *CandidateSet);

  if (getLangOpts().MSVCCompat &&
      CurContext->isDependentContext() && !isSFINAEContext() &&
      (isa<FunctionDecl>(CurContext) || isa<CXXRecordDecl>(CurContext))) {

    OverloadCandidateSet::iterator Best;
    if (CandidateSet->empty() ||
        CandidateSet->BestViableFunction(*this, Fn->getBeginLoc(), Best) ==
            OR_No_Viable_Function) {
      // In Microsoft mode, if we are inside a template class member function
      // then create a type dependent CallExpr. The goal is to postpone name
      // lookup to instantiation time to be able to search into type dependent
      // base classes.
      CallExpr *CE =
          CallExpr::Create(Context, Fn, Args, Context.DependentTy, VK_PRValue,
                           RParenLoc, CurFPFeatureOverrides());
      CE->markDependentForPostponedNameLookup();
      *Result = CE;
      return true;
    }
  }

  if (CandidateSet->empty())
    return false;

  UnbridgedCasts.restore();
  return false;
}

// Guess at what the return type for an unresolvable overload should be.
static QualType chooseRecoveryType(OverloadCandidateSet &CS,
                                   OverloadCandidateSet::iterator *Best) {
  std::optional<QualType> Result;
  // Adjust Type after seeing a candidate.
  auto ConsiderCandidate = [&](const OverloadCandidate &Candidate) {
    if (!Candidate.Function)
      return;
    if (Candidate.Function->isInvalidDecl())
      return;
    QualType T = Candidate.Function->getReturnType();
    if (T.isNull())
      return;
    if (!Result)
      Result = T;
    else if (Result != T)
      Result = QualType();
  };

  // Look for an unambiguous type from a progressively larger subset.
  // e.g. if types disagree, but all *viable* overloads return int, choose int.
  //
  // First, consider only the best candidate.
  if (Best && *Best != CS.end())
    ConsiderCandidate(**Best);
  // Next, consider only viable candidates.
  if (!Result)
    for (const auto &C : CS)
      if (C.Viable)
        ConsiderCandidate(C);
  // Finally, consider all candidates.
  if (!Result)
    for (const auto &C : CS)
      ConsiderCandidate(C);

  if (!Result)
    return QualType();
  auto Value = *Result;
  if (Value.isNull() || Value->isUndeducedType())
    return QualType();
  return Value;
}

/// FinishOverloadedCallExpr - given an OverloadCandidateSet, builds and returns
/// the completed call expression. If overload resolution fails, emits
/// diagnostics and returns ExprError()
static ExprResult FinishOverloadedCallExpr(Sema &SemaRef, Scope *S, Expr *Fn,
                                           UnresolvedLookupExpr *ULE,
                                           SourceLocation LParenLoc,
                                           MultiExprArg Args,
                                           SourceLocation RParenLoc,
                                           Expr *ExecConfig,
                                           OverloadCandidateSet *CandidateSet,
                                           OverloadCandidateSet::iterator *Best,
                                           OverloadingResult OverloadResult,
                                           bool AllowTypoCorrection) {
  switch (OverloadResult) {
  case OR_Success: {
    FunctionDecl *FDecl = (*Best)->Function;
    SemaRef.CheckUnresolvedLookupAccess(ULE, (*Best)->FoundDecl);
    if (SemaRef.DiagnoseUseOfDecl(FDecl, ULE->getNameLoc()))
      return ExprError();
    Fn = SemaRef.FixOverloadedFunctionReference(Fn, (*Best)->FoundDecl, FDecl);
    return SemaRef.BuildResolvedCallExpr(Fn, FDecl, LParenLoc, Args, RParenLoc,
                                         ExecConfig, /*IsExecConfig=*/false,
                                         (*Best)->IsADLCandidate);
  }

  case OR_No_Viable_Function: {
    // Try to recover by looking for viable functions which the user might
    // have meant to call.
    ExprResult Recovery = BuildRecoveryCallExpr(SemaRef, S, Fn, ULE, LParenLoc,
                                                Args, RParenLoc,
                                                CandidateSet->empty(),
                                                AllowTypoCorrection);
    if (Recovery.isInvalid() || Recovery.isUsable())
      return Recovery;

    // If the user passes in a function that we can't take the address of, we
    // generally end up emitting really bad error messages. Here, we attempt to
    // emit better ones.
    for (const Expr *Arg : Args) {
      if (!Arg->getType()->isFunctionType())
        continue;
      if (auto *DRE = dyn_cast<DeclRefExpr>(Arg->IgnoreParenImpCasts())) {
        auto *FD = dyn_cast<FunctionDecl>(DRE->getDecl());
        if (FD &&
            !SemaRef.checkAddressOfFunctionIsAvailable(FD, /*Complain=*/true,
                                                       Arg->getExprLoc()))
          return ExprError();
      }
    }

    CandidateSet->NoteCandidates(
        PartialDiagnosticAt(
            Fn->getBeginLoc(),
            SemaRef.PDiag(diag::err_ovl_no_viable_function_in_call)
                << ULE->getName() << Fn->getSourceRange()),
        SemaRef, OCD_AllCandidates, Args);
    break;
  }

  case OR_Ambiguous:
    CandidateSet->NoteCandidates(
        PartialDiagnosticAt(Fn->getBeginLoc(),
                            SemaRef.PDiag(diag::err_ovl_ambiguous_call)
                                << ULE->getName() << Fn->getSourceRange()),
        SemaRef, OCD_AmbiguousCandidates, Args);
    break;

  case OR_Deleted: {
    CandidateSet->NoteCandidates(
        PartialDiagnosticAt(Fn->getBeginLoc(),
                            SemaRef.PDiag(diag::err_ovl_deleted_call)
                                << ULE->getName() << Fn->getSourceRange()),
        SemaRef, OCD_AllCandidates, Args);

    // We emitted an error for the unavailable/deleted function call but keep
    // the call in the AST.
    FunctionDecl *FDecl = (*Best)->Function;
    Fn = SemaRef.FixOverloadedFunctionReference(Fn, (*Best)->FoundDecl, FDecl);
    return SemaRef.BuildResolvedCallExpr(Fn, FDecl, LParenLoc, Args, RParenLoc,
                                         ExecConfig, /*IsExecConfig=*/false,
                                         (*Best)->IsADLCandidate);
  }
  }

  // Overload resolution failed, try to recover.
  SmallVector<Expr *, 8> SubExprs = {Fn};
  SubExprs.append(Args.begin(), Args.end());
  return SemaRef.CreateRecoveryExpr(Fn->getBeginLoc(), RParenLoc, SubExprs,
                                    chooseRecoveryType(*CandidateSet, Best));
}

static void markUnaddressableCandidatesUnviable(Sema &S,
                                                OverloadCandidateSet &CS) {
  for (auto I = CS.begin(), E = CS.end(); I != E; ++I) {
    if (I->Viable &&
        !S.checkAddressOfFunctionIsAvailable(I->Function, /*Complain=*/false)) {
      I->Viable = false;
      I->FailureKind = ovl_fail_addr_not_available;
    }
  }
}

/// BuildOverloadedCallExpr - Given the call expression that calls Fn
/// (which eventually refers to the declaration Func) and the call
/// arguments Args/NumArgs, attempt to resolve the function call down
/// to a specific function. If overload resolution succeeds, returns
/// the call expression produced by overload resolution.
/// Otherwise, emits diagnostics and returns ExprError.
ExprResult Sema::BuildOverloadedCallExpr(Scope *S, Expr *Fn,
                                         UnresolvedLookupExpr *ULE,
                                         SourceLocation LParenLoc,
                                         MultiExprArg Args,
                                         SourceLocation RParenLoc,
                                         Expr *ExecConfig,
                                         bool AllowTypoCorrection,
                                         bool CalleesAddressIsTaken) {
  OverloadCandidateSet CandidateSet(Fn->getExprLoc(),
                                    OverloadCandidateSet::CSK_Normal);
  ExprResult result;

  if (buildOverloadedCallSet(S, Fn, ULE, Args, LParenLoc, &CandidateSet,
                             &result))
    return result;

  // If the user handed us something like `(&Foo)(Bar)`, we need to ensure that
  // functions that aren't addressible are considered unviable.
  if (CalleesAddressIsTaken)
    markUnaddressableCandidatesUnviable(*this, CandidateSet);

  OverloadCandidateSet::iterator Best;
  OverloadingResult OverloadResult =
      CandidateSet.BestViableFunction(*this, Fn->getBeginLoc(), Best);

  return FinishOverloadedCallExpr(*this, S, Fn, ULE, LParenLoc, Args, RParenLoc,
                                  ExecConfig, &CandidateSet, &Best,
                                  OverloadResult, AllowTypoCorrection);
}

static bool IsOverloaded(const UnresolvedSetImpl &Functions) {
  return Functions.size() > 1 ||
         (Functions.size() == 1 &&
          isa<FunctionTemplateDecl>((*Functions.begin())->getUnderlyingDecl()));
}

ExprResult Sema::CreateUnresolvedLookupExpr(CXXRecordDecl *NamingClass,
                                            NestedNameSpecifierLoc NNSLoc,
                                            DeclarationNameInfo DNI,
                                            const UnresolvedSetImpl &Fns,
                                            bool PerformADL) {
  return UnresolvedLookupExpr::Create(Context, NamingClass, NNSLoc, DNI,
                                      PerformADL, IsOverloaded(Fns),
                                      Fns.begin(), Fns.end());
}

/// Create a unary operation that may resolve to an overloaded
/// operator.
///
/// \param OpLoc The location of the operator itself (e.g., '*').
///
/// \param Opc The UnaryOperatorKind that describes this operator.
///
/// \param Fns The set of non-member functions that will be
/// considered by overload resolution. The caller needs to build this
/// set based on the context using, e.g.,
/// LookupOverloadedOperatorName() and ArgumentDependentLookup(). This
/// set should not contain any member functions; those will be added
/// by CreateOverloadedUnaryOp().
///
/// \param Input The input argument.
ExprResult
Sema::CreateOverloadedUnaryOp(SourceLocation OpLoc, UnaryOperatorKind Opc,
                              const UnresolvedSetImpl &Fns,
                              Expr *Input, bool PerformADL) {
  OverloadedOperatorKind Op = UnaryOperator::getOverloadedOperator(Opc);
  assert(Op != OO_None && "Invalid opcode for overloaded unary operator");
  DeclarationName OpName = Context.DeclarationNames.getCXXOperatorName(Op);
  // TODO: provide better source location info.
  DeclarationNameInfo OpNameInfo(OpName, OpLoc);

  if (checkPlaceholderForOverload(*this, Input))
    return ExprError();

  Expr *Args[2] = { Input, nullptr };
  unsigned NumArgs = 1;

  // For post-increment and post-decrement, add the implicit '0' as
  // the second argument, so that we know this is a post-increment or
  // post-decrement.
  if (Opc == UO_PostInc || Opc == UO_PostDec) {
    llvm::APSInt Zero(Context.getTypeSize(Context.IntTy), false);
    Args[1] = IntegerLiteral::Create(Context, Zero, Context.IntTy,
                                     SourceLocation());
    NumArgs = 2;
  }

  ArrayRef<Expr *> ArgsArray(Args, NumArgs);

  if (Input->isTypeDependent()) {
    if (Fns.empty())
      return UnaryOperator::Create(Context, Input, Opc, Context.DependentTy,
                                   VK_PRValue, OK_Ordinary, OpLoc, false,
                                   CurFPFeatureOverrides());

    CXXRecordDecl *NamingClass = nullptr; // lookup ignores member operators
    ExprResult Fn = CreateUnresolvedLookupExpr(
        NamingClass, NestedNameSpecifierLoc(), OpNameInfo, Fns);
    if (Fn.isInvalid())
      return ExprError();
    return CXXOperatorCallExpr::Create(Context, Op, Fn.get(), ArgsArray,
                                       Context.DependentTy, VK_PRValue, OpLoc,
                                       CurFPFeatureOverrides());
  }

  // Build an empty overload set.
  OverloadCandidateSet CandidateSet(OpLoc, OverloadCandidateSet::CSK_Operator);

  // Add the candidates from the given function set.
  AddNonMemberOperatorCandidates(Fns, ArgsArray, CandidateSet);

  // Add operator candidates that are member functions.
  AddMemberOperatorCandidates(Op, OpLoc, ArgsArray, CandidateSet);

  // Add candidates from ADL.
  if (PerformADL) {
    AddArgumentDependentLookupCandidates(OpName, OpLoc, ArgsArray,
                                         /*ExplicitTemplateArgs*/nullptr,
                                         CandidateSet);
  }

  // Add builtin operator candidates.
  AddBuiltinOperatorCandidates(Op, OpLoc, ArgsArray, CandidateSet);

  bool HadMultipleCandidates = (CandidateSet.size() > 1);

  // Perform overload resolution.
  OverloadCandidateSet::iterator Best;
  switch (CandidateSet.BestViableFunction(*this, OpLoc, Best)) {
  case OR_Success: {
    // We found a built-in operator or an overloaded operator.
    FunctionDecl *FnDecl = Best->Function;

    if (FnDecl) {
      Expr *Base = nullptr;
      // We matched an overloaded operator. Build a call to that
      // operator.

      // Convert the arguments.
      if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(FnDecl)) {
        CheckMemberOperatorAccess(OpLoc, Args[0], nullptr, Best->FoundDecl);

        ExprResult InputRes =
          PerformObjectArgumentInitialization(Input, /*Qualifier=*/nullptr,
                                              Best->FoundDecl, Method);
        if (InputRes.isInvalid())
          return ExprError();
        Base = Input = InputRes.get();
      } else {
        // Convert the arguments.
        ExprResult InputInit
          = PerformCopyInitialization(InitializedEntity::InitializeParameter(
                                                      Context,
                                                      FnDecl->getParamDecl(0)),
                                      SourceLocation(),
                                      Input);
        if (InputInit.isInvalid())
          return ExprError();
        Input = InputInit.get();
      }

      // Build the actual expression node.
      ExprResult FnExpr = CreateFunctionRefExpr(*this, FnDecl, Best->FoundDecl,
                                                Base, HadMultipleCandidates,
                                                OpLoc);
      if (FnExpr.isInvalid())
        return ExprError();

      // Determine the result type.
      QualType ResultTy = FnDecl->getReturnType();
      ExprValueKind VK = Expr::getValueKindForType(ResultTy);
      ResultTy = ResultTy.getNonLValueExprType(Context);

      Args[0] = Input;
      CallExpr *TheCall = CXXOperatorCallExpr::Create(
          Context, Op, FnExpr.get(), ArgsArray, ResultTy, VK, OpLoc,
          CurFPFeatureOverrides(), Best->IsADLCandidate);

      if (CheckCallReturnType(FnDecl->getReturnType(), OpLoc, TheCall, FnDecl))
        return ExprError();

      if (CheckFunctionCall(FnDecl, TheCall,
                            FnDecl->getType()->castAs<FunctionProtoType>()))
        return ExprError();
      return CheckForImmediateInvocation(MaybeBindToTemporary(TheCall), FnDecl);
    } else {
      // We matched a built-in operator. Convert the arguments, then
      // break out so that we will build the appropriate built-in
      // operator node.
      ExprResult InputRes = PerformImplicitConversion(
          Input, Best->BuiltinParamTypes[0], Best->Conversions[0], AA_Passing,
          CCK_ForBuiltinOverloadedOp);
      if (InputRes.isInvalid())
        return ExprError();
      Input = InputRes.get();
      break;
    }
  }

  case OR_No_Viable_Function:
    // This is an erroneous use of an operator which can be overloaded by
    // a non-member function. Check for non-member operators which were
    // defined too late to be candidates.
    if (DiagnoseTwoPhaseOperatorLookup(*this, Op, OpLoc, ArgsArray))
      // FIXME: Recover by calling the found function.
      return ExprError();

    // No viable function; fall through to handling this as a
    // built-in operator, which will produce an error message for us.
    break;

  case OR_Ambiguous:
    CandidateSet.NoteCandidates(
        PartialDiagnosticAt(OpLoc,
                            PDiag(diag::err_ovl_ambiguous_oper_unary)
                                << UnaryOperator::getOpcodeStr(Opc)
                                << Input->getType() << Input->getSourceRange()),
        *this, OCD_AmbiguousCandidates, ArgsArray,
        UnaryOperator::getOpcodeStr(Opc), OpLoc);
    return ExprError();

  case OR_Deleted:
    CandidateSet.NoteCandidates(
        PartialDiagnosticAt(OpLoc, PDiag(diag::err_ovl_deleted_oper)
                                       << UnaryOperator::getOpcodeStr(Opc)
                                       << Input->getSourceRange()),
        *this, OCD_AllCandidates, ArgsArray, UnaryOperator::getOpcodeStr(Opc),
        OpLoc);
    return ExprError();
  }

  // Either we found no viable overloaded operator or we matched a
  // built-in operator. In either case, fall through to trying to
  // build a built-in operation.
  return CreateBuiltinUnaryOp(OpLoc, Opc, Input);
}

/// Perform lookup for an overloaded binary operator.
void Sema::LookupOverloadedBinOp(OverloadCandidateSet &CandidateSet,
                                 OverloadedOperatorKind Op,
                                 const UnresolvedSetImpl &Fns,
                                 ArrayRef<Expr *> Args, bool PerformADL) {
  SourceLocation OpLoc = CandidateSet.getLocation();

  OverloadedOperatorKind ExtraOp =
      CandidateSet.getRewriteInfo().AllowRewrittenCandidates
          ? getRewrittenOverloadedOperator(Op)
          : OO_None;

  // Add the candidates from the given function set. This also adds the
  // rewritten candidates using these functions if necessary.
  AddNonMemberOperatorCandidates(Fns, Args, CandidateSet);

  // Add operator candidates that are member functions.
  AddMemberOperatorCandidates(Op, OpLoc, Args, CandidateSet);
  if (CandidateSet.getRewriteInfo().allowsReversed(Op))
    AddMemberOperatorCandidates(Op, OpLoc, {Args[1], Args[0]}, CandidateSet,
                                OverloadCandidateParamOrder::Reversed);

  // In C++20, also add any rewritten member candidates.
  if (ExtraOp) {
    AddMemberOperatorCandidates(ExtraOp, OpLoc, Args, CandidateSet);
    if (CandidateSet.getRewriteInfo().allowsReversed(ExtraOp))
      AddMemberOperatorCandidates(ExtraOp, OpLoc, {Args[1], Args[0]},
                                  CandidateSet,
                                  OverloadCandidateParamOrder::Reversed);
  }

  // Add candidates from ADL. Per [over.match.oper]p2, this lookup is not
  // performed for an assignment operator (nor for operator[] nor operator->,
  // which don't get here).
  if (Op != OO_Equal && PerformADL) {
    DeclarationName OpName = Context.DeclarationNames.getCXXOperatorName(Op);
    AddArgumentDependentLookupCandidates(OpName, OpLoc, Args,
                                         /*ExplicitTemplateArgs*/ nullptr,
                                         CandidateSet);
    if (ExtraOp) {
      DeclarationName ExtraOpName =
          Context.DeclarationNames.getCXXOperatorName(ExtraOp);
      AddArgumentDependentLookupCandidates(ExtraOpName, OpLoc, Args,
                                           /*ExplicitTemplateArgs*/ nullptr,
                                           CandidateSet);
    }
  }

  // Add builtin operator candidates.
  //
  // FIXME: We don't add any rewritten candidates here. This is strictly
  // incorrect; a builtin candidate could be hidden by a non-viable candidate,
  // resulting in our selecting a rewritten builtin candidate. For example:
  //
  //   enum class E { e };
  //   bool operator!=(E, E) requires false;
  //   bool k = E::e != E::e;
  //
  // ... should select the rewritten builtin candidate 'operator==(E, E)'. But
  // it seems unreasonable to consider rewritten builtin candidates. A core
  // issue has been filed proposing to removed this requirement.
  AddBuiltinOperatorCandidates(Op, OpLoc, Args, CandidateSet);
}

/// Create a binary operation that may resolve to an overloaded
/// operator.
///
/// \param OpLoc The location of the operator itself (e.g., '+').
///
/// \param Opc The BinaryOperatorKind that describes this operator.
///
/// \param Fns The set of non-member functions that will be
/// considered by overload resolution. The caller needs to build this
/// set based on the context using, e.g.,
/// LookupOverloadedOperatorName() and ArgumentDependentLookup(). This
/// set should not contain any member functions; those will be added
/// by CreateOverloadedBinOp().
///
/// \param LHS Left-hand argument.
/// \param RHS Right-hand argument.
/// \param PerformADL Whether to consider operator candidates found by ADL.
/// \param AllowRewrittenCandidates Whether to consider candidates found by
///        C++20 operator rewrites.
/// \param DefaultedFn If we are synthesizing a defaulted operator function,
///        the function in question. Such a function is never a candidate in
///        our overload resolution. This also enables synthesizing a three-way
///        comparison from < and == as described in C++20 [class.spaceship]p1.
ExprResult Sema::CreateOverloadedBinOp(SourceLocation OpLoc,
                                       BinaryOperatorKind Opc,
                                       const UnresolvedSetImpl &Fns, Expr *LHS,
                                       Expr *RHS, bool PerformADL,
                                       bool AllowRewrittenCandidates,
                                       FunctionDecl *DefaultedFn) {
  Expr *Args[2] = { LHS, RHS };
  LHS=RHS=nullptr; // Please use only Args instead of LHS/RHS couple

  if (!getLangOpts().CPlusPlus20)
    AllowRewrittenCandidates = false;

  OverloadedOperatorKind Op = BinaryOperator::getOverloadedOperator(Opc);

  // If either side is type-dependent, create an appropriate dependent
  // expression.
  if (Args[0]->isTypeDependent() || Args[1]->isTypeDependent()) {
    if (Fns.empty()) {
      // If there are no functions to store, just build a dependent
      // BinaryOperator or CompoundAssignment.
      if (BinaryOperator::isCompoundAssignmentOp(Opc))
        return CompoundAssignOperator::Create(
            Context, Args[0], Args[1], Opc, Context.DependentTy, VK_LValue,
            OK_Ordinary, OpLoc, CurFPFeatureOverrides(), Context.DependentTy,
            Context.DependentTy);
      return BinaryOperator::Create(
          Context, Args[0], Args[1], Opc, Context.DependentTy, VK_PRValue,
          OK_Ordinary, OpLoc, CurFPFeatureOverrides());
    }

    // FIXME: save results of ADL from here?
    CXXRecordDecl *NamingClass = nullptr; // lookup ignores member operators
    // TODO: provide better source location info in DNLoc component.
    DeclarationName OpName = Context.DeclarationNames.getCXXOperatorName(Op);
    DeclarationNameInfo OpNameInfo(OpName, OpLoc);
    ExprResult Fn = CreateUnresolvedLookupExpr(
        NamingClass, NestedNameSpecifierLoc(), OpNameInfo, Fns, PerformADL);
    if (Fn.isInvalid())
      return ExprError();
    return CXXOperatorCallExpr::Create(Context, Op, Fn.get(), Args,
                                       Context.DependentTy, VK_PRValue, OpLoc,
                                       CurFPFeatureOverrides());
  }

  // Always do placeholder-like conversions on the RHS.
  if (checkPlaceholderForOverload(*this, Args[1]))
    return ExprError();

  // Do placeholder-like conversion on the LHS; note that we should
  // not get here with a PseudoObject LHS.
  assert(Args[0]->getObjectKind() != OK_ObjCProperty);
  if (checkPlaceholderForOverload(*this, Args[0]))
    return ExprError();

  // If this is the assignment operator, we only perform overload resolution
  // if the left-hand side is a class or enumeration type. This is actually
  // a hack. The standard requires that we do overload resolution between the
  // various built-in candidates, but as DR507 points out, this can lead to
  // problems. So we do it this way, which pretty much follows what GCC does.
  // Note that we go the traditional code path for compound assignment forms.
  if (Opc == BO_Assign && !Args[0]->getType()->isOverloadableType())
    return CreateBuiltinBinOp(OpLoc, Opc, Args[0], Args[1]);

  // If this is the .* operator, which is not overloadable, just
  // create a built-in binary operator.
  if (Opc == BO_PtrMemD)
    return CreateBuiltinBinOp(OpLoc, Opc, Args[0], Args[1]);

  // Build the overload set.
  OverloadCandidateSet CandidateSet(OpLoc, OverloadCandidateSet::CSK_Operator,
                                    OverloadCandidateSet::OperatorRewriteInfo(
                                        Op, OpLoc, AllowRewrittenCandidates));
  if (DefaultedFn)
    CandidateSet.exclude(DefaultedFn);
  LookupOverloadedBinOp(CandidateSet, Op, Fns, Args, PerformADL);

  bool HadMultipleCandidates = (CandidateSet.size() > 1);

  // Perform overload resolution.
  OverloadCandidateSet::iterator Best;
  switch (CandidateSet.BestViableFunction(*this, OpLoc, Best)) {
    case OR_Success: {
      // We found a built-in operator or an overloaded operator.
      FunctionDecl *FnDecl = Best->Function;

      bool IsReversed = Best->isReversed();
      if (IsReversed)
        std::swap(Args[0], Args[1]);

      if (FnDecl) {
        Expr *Base = nullptr;
        // We matched an overloaded operator. Build a call to that
        // operator.

        OverloadedOperatorKind ChosenOp =
            FnDecl->getDeclName().getCXXOverloadedOperator();

        // C++2a [over.match.oper]p9:
        //   If a rewritten operator== candidate is selected by overload
        //   resolution for an operator@, its return type shall be cv bool
        if (Best->RewriteKind && ChosenOp == OO_EqualEqual &&
            !FnDecl->getReturnType()->isBooleanType()) {
          bool IsExtension =
              FnDecl->getReturnType()->isIntegralOrUnscopedEnumerationType();
          Diag(OpLoc, IsExtension ? diag::ext_ovl_rewrite_equalequal_not_bool
                                  : diag::err_ovl_rewrite_equalequal_not_bool)
              << FnDecl->getReturnType() << BinaryOperator::getOpcodeStr(Opc)
              << Args[0]->getSourceRange() << Args[1]->getSourceRange();
          Diag(FnDecl->getLocation(), diag::note_declared_at);
          if (!IsExtension)
            return ExprError();
        }

        if (AllowRewrittenCandidates && !IsReversed &&
            CandidateSet.getRewriteInfo().isReversible()) {
          // We could have reversed this operator, but didn't. Check if some
          // reversed form was a viable candidate, and if so, if it had a
          // better conversion for either parameter. If so, this call is
          // formally ambiguous, and allowing it is an extension.
          llvm::SmallVector<FunctionDecl*, 4> AmbiguousWith;
          for (OverloadCandidate &Cand : CandidateSet) {
            if (Cand.Viable && Cand.Function && Cand.isReversed() &&
                haveSameParameterTypes(Context, Cand.Function, FnDecl, 2)) {
              for (unsigned ArgIdx = 0; ArgIdx < 2; ++ArgIdx) {
                if (CompareImplicitConversionSequences(
                        *this, OpLoc, Cand.Conversions[ArgIdx],
                        Best->Conversions[ArgIdx]) ==
                    ImplicitConversionSequence::Better) {
                  AmbiguousWith.push_back(Cand.Function);
                  break;
                }
              }
            }
          }

          if (!AmbiguousWith.empty()) {
            bool AmbiguousWithSelf =
                AmbiguousWith.size() == 1 &&
                declaresSameEntity(AmbiguousWith.front(), FnDecl);
            Diag(OpLoc, diag::ext_ovl_ambiguous_oper_binary_reversed)
                << BinaryOperator::getOpcodeStr(Opc)
                << Args[0]->getType() << Args[1]->getType() << AmbiguousWithSelf
                << Args[0]->getSourceRange() << Args[1]->getSourceRange();
            if (AmbiguousWithSelf) {
              Diag(FnDecl->getLocation(),
                   diag::note_ovl_ambiguous_oper_binary_reversed_self);
              // Mark member== const or provide matching != to disallow reversed
              // args. Eg. 
              // struct S { bool operator==(const S&); }; 
              // S()==S();
              if (auto *MD = dyn_cast<CXXMethodDecl>(FnDecl))
                if (Op == OverloadedOperatorKind::OO_EqualEqual &&
                    !MD->isConst() &&
                    Context.hasSameUnqualifiedType(
                        MD->getThisObjectType(),
                        MD->getParamDecl(0)->getType().getNonReferenceType()) &&
                    Context.hasSameUnqualifiedType(MD->getThisObjectType(),
                                                   Args[0]->getType()) &&
                    Context.hasSameUnqualifiedType(MD->getThisObjectType(),
                                                   Args[1]->getType()))
                  Diag(FnDecl->getLocation(),
                       diag::note_ovl_ambiguous_eqeq_reversed_self_non_const);
            } else {
              Diag(FnDecl->getLocation(),
                   diag::note_ovl_ambiguous_oper_binary_selected_candidate);
              for (auto *F : AmbiguousWith)
                Diag(F->getLocation(),
                     diag::note_ovl_ambiguous_oper_binary_reversed_candidate);
            }
          }
        }

        // Convert the arguments.
        if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(FnDecl)) {
          // Best->Access is only meaningful for class members.
          CheckMemberOperatorAccess(OpLoc, Args[0], Args[1], Best->FoundDecl);

          ExprResult Arg1 =
            PerformCopyInitialization(
              InitializedEntity::InitializeParameter(Context,
                                                     FnDecl->getParamDecl(0)),
              SourceLocation(), Args[1]);
          if (Arg1.isInvalid())
            return ExprError();

          ExprResult Arg0 =
            PerformObjectArgumentInitialization(Args[0], /*Qualifier=*/nullptr,
                                                Best->FoundDecl, Method);
          if (Arg0.isInvalid())
            return ExprError();
          Base = Args[0] = Arg0.getAs<Expr>();
          Args[1] = RHS = Arg1.getAs<Expr>();
        } else {
          // Convert the arguments.
          ExprResult Arg0 = PerformCopyInitialization(
            InitializedEntity::InitializeParameter(Context,
                                                   FnDecl->getParamDecl(0)),
            SourceLocation(), Args[0]);
          if (Arg0.isInvalid())
            return ExprError();

          ExprResult Arg1 =
            PerformCopyInitialization(
              InitializedEntity::InitializeParameter(Context,
                                                     FnDecl->getParamDecl(1)),
              SourceLocation(), Args[1]);
          if (Arg1.isInvalid())
            return ExprError();
          Args[0] = LHS = Arg0.getAs<Expr>();
          Args[1] = RHS = Arg1.getAs<Expr>();
        }

        // Build the actual expression node.
        ExprResult FnExpr = CreateFunctionRefExpr(*this, FnDecl,
                                                  Best->FoundDecl, Base,
                                                  HadMultipleCandidates, OpLoc);
        if (FnExpr.isInvalid())
          return ExprError();

        // Determine the result type.
        QualType ResultTy = FnDecl->getReturnType();
        ExprValueKind VK = Expr::getValueKindForType(ResultTy);
        ResultTy = ResultTy.getNonLValueExprType(Context);

        CXXOperatorCallExpr *TheCall = CXXOperatorCallExpr::Create(
            Context, ChosenOp, FnExpr.get(), Args, ResultTy, VK, OpLoc,
            CurFPFeatureOverrides(), Best->IsADLCandidate);

        if (CheckCallReturnType(FnDecl->getReturnType(), OpLoc, TheCall,
                                FnDecl))
          return ExprError();

        ArrayRef<const Expr *> ArgsArray(Args, 2);
        const Expr *ImplicitThis = nullptr;
        // Cut off the implicit 'this'.
        if (isa<CXXMethodDecl>(FnDecl)) {
          ImplicitThis = ArgsArray[0];
          ArgsArray = ArgsArray.slice(1);
        }

        // Check for a self move.
        if (Op == OO_Equal)
          DiagnoseSelfMove(Args[0], Args[1], OpLoc);

        if (ImplicitThis) {
          QualType ThisType = Context.getPointerType(ImplicitThis->getType());
          QualType ThisTypeFromDecl = Context.getPointerType(
              cast<CXXMethodDecl>(FnDecl)->getThisObjectType());

          CheckArgAlignment(OpLoc, FnDecl, "'this'", ThisType,
                            ThisTypeFromDecl);
        }

        checkCall(FnDecl, nullptr, ImplicitThis, ArgsArray,
                  isa<CXXMethodDecl>(FnDecl), OpLoc, TheCall->getSourceRange(),
                  VariadicDoesNotApply);

        ExprResult R = MaybeBindToTemporary(TheCall);
        if (R.isInvalid())
          return ExprError();

        R = CheckForImmediateInvocation(R, FnDecl);
        if (R.isInvalid())
          return ExprError();

        // For a rewritten candidate, we've already reversed the arguments
        // if needed. Perform the rest of the rewrite now.
        if ((Best->RewriteKind & CRK_DifferentOperator) ||
            (Op == OO_Spaceship && IsReversed)) {
          if (Op == OO_ExclaimEqual) {
            assert(ChosenOp == OO_EqualEqual && "unexpected operator name");
            R = CreateBuiltinUnaryOp(OpLoc, UO_LNot, R.get());
          } else {
            assert(ChosenOp == OO_Spaceship && "unexpected operator name");
            llvm::APSInt Zero(Context.getTypeSize(Context.IntTy), false);
            Expr *ZeroLiteral =
                IntegerLiteral::Create(Context, Zero, Context.IntTy, OpLoc);

            Sema::CodeSynthesisContext Ctx;
            Ctx.Kind = Sema::CodeSynthesisContext::RewritingOperatorAsSpaceship;
            Ctx.Entity = FnDecl;
            pushCodeSynthesisContext(Ctx);

            R = CreateOverloadedBinOp(
                OpLoc, Opc, Fns, IsReversed ? ZeroLiteral : R.get(),
                IsReversed ? R.get() : ZeroLiteral, /*PerformADL=*/true,
                /*AllowRewrittenCandidates=*/false);

            popCodeSynthesisContext();
          }
          if (R.isInvalid())
            return ExprError();
        } else {
          assert(ChosenOp == Op && "unexpected operator name");
        }

        // Make a note in the AST if we did any rewriting.
        if (Best->RewriteKind != CRK_None)
          R = new (Context) CXXRewrittenBinaryOperator(R.get(), IsReversed);

        return R;
      } else {
        // We matched a built-in operator. Convert the arguments, then
        // break out so that we will build the appropriate built-in
        // operator node.
        ExprResult ArgsRes0 = PerformImplicitConversion(
            Args[0], Best->BuiltinParamTypes[0], Best->Conversions[0],
            AA_Passing, CCK_ForBuiltinOverloadedOp);
        if (ArgsRes0.isInvalid())
          return ExprError();
        Args[0] = ArgsRes0.get();

        ExprResult ArgsRes1 = PerformImplicitConversion(
            Args[1], Best->BuiltinParamTypes[1], Best->Conversions[1],
            AA_Passing, CCK_ForBuiltinOverloadedOp);
        if (ArgsRes1.isInvalid())
          return ExprError();
        Args[1] = ArgsRes1.get();
        break;
      }
    }

    case OR_No_Viable_Function: {
      // C++ [over.match.oper]p9:
      //   If the operator is the operator , [...] and there are no
      //   viable functions, then the operator is assumed to be the
      //   built-in operator and interpreted according to clause 5.
      if (Opc == BO_Comma)
        break;

      // When defaulting an 'operator<=>', we can try to synthesize a three-way
      // compare result using '==' and '<'.
      if (DefaultedFn && Opc == BO_Cmp) {
        ExprResult E = BuildSynthesizedThreeWayComparison(OpLoc, Fns, Args[0],
                                                          Args[1], DefaultedFn);
        if (E.isInvalid() || E.isUsable())
          return E;
      }

      // For class as left operand for assignment or compound assignment
      // operator do not fall through to handling in built-in, but report that
      // no overloaded assignment operator found
      ExprResult Result = ExprError();
      StringRef OpcStr = BinaryOperator::getOpcodeStr(Opc);
      auto Cands = CandidateSet.CompleteCandidates(*this, OCD_AllCandidates,
                                                   Args, OpLoc);
      DeferDiagsRAII DDR(*this,
                         CandidateSet.shouldDeferDiags(*this, Args, OpLoc));
      if (Args[0]->getType()->isRecordType() &&
          Opc >= BO_Assign && Opc <= BO_OrAssign) {
        Diag(OpLoc,  diag::err_ovl_no_viable_oper)
             << BinaryOperator::getOpcodeStr(Opc)
             << Args[0]->getSourceRange() << Args[1]->getSourceRange();
        if (Args[0]->getType()->isIncompleteType()) {
          Diag(OpLoc, diag::note_assign_lhs_incomplete)
            << Args[0]->getType()
            << Args[0]->getSourceRange() << Args[1]->getSourceRange();
        }
      } else {
        // This is an erroneous use of an operator which can be overloaded by
        // a non-member function. Check for non-member operators which were
        // defined too late to be candidates.
        if (DiagnoseTwoPhaseOperatorLookup(*this, Op, OpLoc, Args))
          // FIXME: Recover by calling the found function.
          return ExprError();

        // No viable function; try to create a built-in operation, which will
        // produce an error. Then, show the non-viable candidates.
        Result = CreateBuiltinBinOp(OpLoc, Opc, Args[0], Args[1]);
      }
      assert(Result.isInvalid() &&
             "C++ binary operator overloading is missing candidates!");
      CandidateSet.NoteCandidates(*this, Args, Cands, OpcStr, OpLoc);
      return Result;
    }

    case OR_Ambiguous:
      CandidateSet.NoteCandidates(
          PartialDiagnosticAt(OpLoc, PDiag(diag::err_ovl_ambiguous_oper_binary)
                                         << BinaryOperator::getOpcodeStr(Opc)
                                         << Args[0]->getType()
                                         << Args[1]->getType()
                                         << Args[0]->getSourceRange()
                                         << Args[1]->getSourceRange()),
          *this, OCD_AmbiguousCandidates, Args, BinaryOperator::getOpcodeStr(Opc),
          OpLoc);
      return ExprError();

    case OR_Deleted:
      if (isImplicitlyDeleted(Best->Function)) {
        FunctionDecl *DeletedFD = Best->Function;
        DefaultedFunctionKind DFK = getDefaultedFunctionKind(DeletedFD);
        if (DFK.isSpecialMember()) {
          Diag(OpLoc, diag::err_ovl_deleted_special_oper)
            << Args[0]->getType() << DFK.asSpecialMember();
        } else {
          assert(DFK.isComparison());
          Diag(OpLoc, diag::err_ovl_deleted_comparison)
            << Args[0]->getType() << DeletedFD;
        }

        // The user probably meant to call this special member. Just
        // explain why it's deleted.
        NoteDeletedFunction(DeletedFD);
        return ExprError();
      }
      CandidateSet.NoteCandidates(
          PartialDiagnosticAt(
              OpLoc, PDiag(diag::err_ovl_deleted_oper)
                         << getOperatorSpelling(Best->Function->getDeclName()
                                                    .getCXXOverloadedOperator())
                         << Args[0]->getSourceRange()
                         << Args[1]->getSourceRange()),
          *this, OCD_AllCandidates, Args, BinaryOperator::getOpcodeStr(Opc),
          OpLoc);
      return ExprError();
  }

  // We matched a built-in operator; build it.
  return CreateBuiltinBinOp(OpLoc, Opc, Args[0], Args[1]);
}

ExprResult Sema::BuildSynthesizedThreeWayComparison(
    SourceLocation OpLoc, const UnresolvedSetImpl &Fns, Expr *LHS, Expr *RHS,
    FunctionDecl *DefaultedFn) {
  const ComparisonCategoryInfo *Info =
      Context.CompCategories.lookupInfoForType(DefaultedFn->getReturnType());
  // If we're not producing a known comparison category type, we can't
  // synthesize a three-way comparison. Let the caller diagnose this.
  if (!Info)
    return ExprResult((Expr*)nullptr);

  // If we ever want to perform this synthesis more generally, we will need to
  // apply the temporary materialization conversion to the operands.
  assert(LHS->isGLValue() && RHS->isGLValue() &&
         "cannot use prvalue expressions more than once");
  Expr *OrigLHS = LHS;
  Expr *OrigRHS = RHS;

  // Replace the LHS and RHS with OpaqueValueExprs; we're going to refer to
  // each of them multiple times below.
  LHS = new (Context)
      OpaqueValueExpr(LHS->getExprLoc(), LHS->getType(), LHS->getValueKind(),
                      LHS->getObjectKind(), LHS);
  RHS = new (Context)
      OpaqueValueExpr(RHS->getExprLoc(), RHS->getType(), RHS->getValueKind(),
                      RHS->getObjectKind(), RHS);

  ExprResult Eq = CreateOverloadedBinOp(OpLoc, BO_EQ, Fns, LHS, RHS, true, true,
                                        DefaultedFn);
  if (Eq.isInvalid())
    return ExprError();

  ExprResult Less = CreateOverloadedBinOp(OpLoc, BO_LT, Fns, LHS, RHS, true,
                                          true, DefaultedFn);
  if (Less.isInvalid())
    return ExprError();

  ExprResult Greater;
  if (Info->isPartial()) {
    Greater = CreateOverloadedBinOp(OpLoc, BO_LT, Fns, RHS, LHS, true, true,
                                    DefaultedFn);
    if (Greater.isInvalid())
      return ExprError();
  }

  // Form the list of comparisons we're going to perform.
  struct Comparison {
    ExprResult Cmp;
    ComparisonCategoryResult Result;
  } Comparisons[4] =
  { {Eq, Info->isStrong() ? ComparisonCategoryResult::Equal
                          : ComparisonCategoryResult::Equivalent},
    {Less, ComparisonCategoryResult::Less},
    {Greater, ComparisonCategoryResult::Greater},
    {ExprResult(), ComparisonCategoryResult::Unordered},
  };

  int I = Info->isPartial() ? 3 : 2;

  // Combine the comparisons with suitable conditional expressions.
  ExprResult Result;
  for (; I >= 0; --I) {
    // Build a reference to the comparison category constant.
    auto *VI = Info->lookupValueInfo(Comparisons[I].Result);
    // FIXME: Missing a constant for a comparison category. Diagnose this?
    if (!VI)
      return ExprResult((Expr*)nullptr);
    ExprResult ThisResult =
        BuildDeclarationNameExpr(CXXScopeSpec(), DeclarationNameInfo(), VI->VD);
    if (ThisResult.isInvalid())
      return ExprError();

    // Build a conditional unless this is the final case.
    if (Result.get()) {
      Result = ActOnConditionalOp(OpLoc, OpLoc, Comparisons[I].Cmp.get(),
                                  ThisResult.get(), Result.get());
      if (Result.isInvalid())
        return ExprError();
    } else {
      Result = ThisResult;
    }
  }

  // Build a PseudoObjectExpr to model the rewriting of an <=> operator, and to
  // bind the OpaqueValueExprs before they're (repeatedly) used.
  Expr *SyntacticForm = BinaryOperator::Create(
      Context, OrigLHS, OrigRHS, BO_Cmp, Result.get()->getType(),
      Result.get()->getValueKind(), Result.get()->getObjectKind(), OpLoc,
      CurFPFeatureOverrides());
  Expr *SemanticForm[] = {LHS, RHS, Result.get()};
  return PseudoObjectExpr::Create(Context, SyntacticForm, SemanticForm, 2);
}

static bool PrepareArgumentsForCallToObjectOfClassType(
    Sema &S, SmallVectorImpl<Expr *> &MethodArgs, CXXMethodDecl *Method,
    MultiExprArg Args, SourceLocation LParenLoc) {

  const auto *Proto = Method->getType()->castAs<FunctionProtoType>();
  unsigned NumParams = Proto->getNumParams();
  unsigned NumArgsSlots =
      MethodArgs.size() + std::max<unsigned>(Args.size(), NumParams);
  // Build the full argument list for the method call (the implicit object
  // parameter is placed at the beginning of the list).
  MethodArgs.reserve(MethodArgs.size() + NumArgsSlots);
  bool IsError = false;
  // Initialize the implicit object parameter.
  // Check the argument types.
  for (unsigned i = 0; i != NumParams; i++) {
    Expr *Arg;
    if (i < Args.size()) {
      Arg = Args[i];
      ExprResult InputInit =
          S.PerformCopyInitialization(InitializedEntity::InitializeParameter(
                                          S.Context, Method->getParamDecl(i)),
                                      SourceLocation(), Arg);
      IsError |= InputInit.isInvalid();
      Arg = InputInit.getAs<Expr>();
    } else {
      ExprResult DefArg =
          S.BuildCXXDefaultArgExpr(LParenLoc, Method, Method->getParamDecl(i));
      if (DefArg.isInvalid()) {
        IsError = true;
        break;
      }
      Arg = DefArg.getAs<Expr>();
    }

    MethodArgs.push_back(Arg);
  }
  return IsError;
}

ExprResult Sema::CreateOverloadedArraySubscriptExpr(SourceLocation LLoc,
                                                    SourceLocation RLoc,
                                                    Expr *Base,
                                                    MultiExprArg ArgExpr) {
  SmallVector<Expr *, 2> Args;
  Args.push_back(Base);
  for (auto *e : ArgExpr) {
    Args.push_back(e);
  }
  DeclarationName OpName =
      Context.DeclarationNames.getCXXOperatorName(OO_Subscript);

  SourceRange Range = ArgExpr.empty()
                          ? SourceRange{}
                          : SourceRange(ArgExpr.front()->getBeginLoc(),
                                        ArgExpr.back()->getEndLoc());

  // If either side is type-dependent, create an appropriate dependent
  // expression.
  if (Expr::hasAnyTypeDependentArguments(Args)) {

    CXXRecordDecl *NamingClass = nullptr; // lookup ignores member operators
    // CHECKME: no 'operator' keyword?
    DeclarationNameInfo OpNameInfo(OpName, LLoc);
    OpNameInfo.setCXXOperatorNameRange(SourceRange(LLoc, RLoc));
    ExprResult Fn = CreateUnresolvedLookupExpr(
        NamingClass, NestedNameSpecifierLoc(), OpNameInfo, UnresolvedSet<0>());
    if (Fn.isInvalid())
      return ExprError();
    // Can't add any actual overloads yet

    return CXXOperatorCallExpr::Create(Context, OO_Subscript, Fn.get(), Args,
                                       Context.DependentTy, VK_PRValue, RLoc,
                                       CurFPFeatureOverrides());
  }

  // Handle placeholders
  UnbridgedCastsSet UnbridgedCasts;
  if (checkArgPlaceholdersForOverload(*this, Args, UnbridgedCasts)) {
    return ExprError();
  }
  // Build an empty overload set.
  OverloadCandidateSet CandidateSet(LLoc, OverloadCandidateSet::CSK_Operator);

  // Subscript can only be overloaded as a member function.

  // Add operator candidates that are member functions.
  AddMemberOperatorCandidates(OO_Subscript, LLoc, Args, CandidateSet);

  // Add builtin operator candidates.
  if (Args.size() == 2)
    AddBuiltinOperatorCandidates(OO_Subscript, LLoc, Args, CandidateSet);

  bool HadMultipleCandidates = (CandidateSet.size() > 1);

  // Perform overload resolution.
  OverloadCandidateSet::iterator Best;
  switch (CandidateSet.BestViableFunction(*this, LLoc, Best)) {
    case OR_Success: {
      // We found a built-in operator or an overloaded operator.
      FunctionDecl *FnDecl = Best->Function;

      if (FnDecl) {
        // We matched an overloaded operator. Build a call to that
        // operator.

        CheckMemberOperatorAccess(LLoc, Args[0], ArgExpr, Best->FoundDecl);

        // Convert the arguments.
        CXXMethodDecl *Method = cast<CXXMethodDecl>(FnDecl);
        SmallVector<Expr *, 2> MethodArgs;

        // Handle 'this' parameter if the selected function is not static.
        if (Method->isInstance()) {
          ExprResult Arg0 = PerformObjectArgumentInitialization(
              Args[0], /*Qualifier=*/nullptr, Best->FoundDecl, Method);
          if (Arg0.isInvalid())
            return ExprError();

          MethodArgs.push_back(Arg0.get());
        }

        bool IsError = PrepareArgumentsForCallToObjectOfClassType(
            *this, MethodArgs, Method, ArgExpr, LLoc);
        if (IsError)
          return ExprError();

        // Build the actual expression node.
        DeclarationNameInfo OpLocInfo(OpName, LLoc);
        OpLocInfo.setCXXOperatorNameRange(SourceRange(LLoc, RLoc));
        ExprResult FnExpr = CreateFunctionRefExpr(
            *this, FnDecl, Best->FoundDecl, Base, HadMultipleCandidates,
            OpLocInfo.getLoc(), OpLocInfo.getInfo());
        if (FnExpr.isInvalid())
          return ExprError();

        // Determine the result type
        QualType ResultTy = FnDecl->getReturnType();
        ExprValueKind VK = Expr::getValueKindForType(ResultTy);
        ResultTy = ResultTy.getNonLValueExprType(Context);

        CallExpr *TheCall;
        if (Method->isInstance())
          TheCall = CXXOperatorCallExpr::Create(
              Context, OO_Subscript, FnExpr.get(), MethodArgs, ResultTy, VK,
              RLoc, CurFPFeatureOverrides());
        else
          TheCall =
              CallExpr::Create(Context, FnExpr.get(), MethodArgs, ResultTy, VK,
                               RLoc, CurFPFeatureOverrides());

        if (CheckCallReturnType(FnDecl->getReturnType(), LLoc, TheCall, FnDecl))
          return ExprError();

        if (CheckFunctionCall(Method, TheCall,
                              Method->getType()->castAs<FunctionProtoType>()))
          return ExprError();

        return CheckForImmediateInvocation(MaybeBindToTemporary(TheCall),
                                           FnDecl);
      } else {
        // We matched a built-in operator. Convert the arguments, then
        // break out so that we will build the appropriate built-in
        // operator node.
        ExprResult ArgsRes0 = PerformImplicitConversion(
            Args[0], Best->BuiltinParamTypes[0], Best->Conversions[0],
            AA_Passing, CCK_ForBuiltinOverloadedOp);
        if (ArgsRes0.isInvalid())
          return ExprError();
        Args[0] = ArgsRes0.get();

        ExprResult ArgsRes1 = PerformImplicitConversion(
            Args[1], Best->BuiltinParamTypes[1], Best->Conversions[1],
            AA_Passing, CCK_ForBuiltinOverloadedOp);
        if (ArgsRes1.isInvalid())
          return ExprError();
        Args[1] = ArgsRes1.get();

        break;
      }
    }

    case OR_No_Viable_Function: {
      PartialDiagnostic PD =
          CandidateSet.empty()
              ? (PDiag(diag::err_ovl_no_oper)
                 << Args[0]->getType() << /*subscript*/ 0
                 << Args[0]->getSourceRange() << Range)
              : (PDiag(diag::err_ovl_no_viable_subscript)
                 << Args[0]->getType() << Args[0]->getSourceRange() << Range);
      CandidateSet.NoteCandidates(PartialDiagnosticAt(LLoc, PD), *this,
                                  OCD_AllCandidates, ArgExpr, "[]", LLoc);
      return ExprError();
    }

    case OR_Ambiguous:
      if (Args.size() == 2) {
        CandidateSet.NoteCandidates(
            PartialDiagnosticAt(
                LLoc, PDiag(diag::err_ovl_ambiguous_oper_binary)
                          << "[]" << Args[0]->getType() << Args[1]->getType()
                          << Args[0]->getSourceRange() << Range),
            *this, OCD_AmbiguousCandidates, Args, "[]", LLoc);
      } else {
        CandidateSet.NoteCandidates(
            PartialDiagnosticAt(LLoc,
                                PDiag(diag::err_ovl_ambiguous_subscript_call)
                                    << Args[0]->getType()
                                    << Args[0]->getSourceRange() << Range),
            *this, OCD_AmbiguousCandidates, Args, "[]", LLoc);
      }
      return ExprError();

    case OR_Deleted:
      CandidateSet.NoteCandidates(
          PartialDiagnosticAt(LLoc, PDiag(diag::err_ovl_deleted_oper)
                                        << "[]" << Args[0]->getSourceRange()
                                        << Range),
          *this, OCD_AllCandidates, Args, "[]", LLoc);
      return ExprError();
    }

  // We matched a built-in operator; build it.
  return CreateBuiltinArraySubscriptExpr(Args[0], LLoc, Args[1], RLoc);
}

/// BuildCallToMemberFunction - Build a call to a member
/// function. MemExpr is the expression that refers to the member
/// function (and includes the object parameter), Args/NumArgs are the
/// arguments to the function call (not including the object
/// parameter). The caller needs to validate that the member
/// expression refers to a non-static member function or an overloaded
/// member function.
ExprResult Sema::BuildCallToMemberFunction(Scope *S, Expr *MemExprE,
                                           SourceLocation LParenLoc,
                                           MultiExprArg Args,
                                           SourceLocation RParenLoc,
                                           Expr *ExecConfig, bool IsExecConfig,
                                           bool AllowRecovery) {
  assert(MemExprE->getType() == Context.BoundMemberTy ||
         MemExprE->getType() == Context.OverloadTy);

  // Dig out the member expression. This holds both the object
  // argument and the member function we're referring to.
  Expr *NakedMemExpr = MemExprE->IgnoreParens();

  // Determine whether this is a call to a pointer-to-member function.
  if (BinaryOperator *op = dyn_cast<BinaryOperator>(NakedMemExpr)) {
    assert(op->getType() == Context.BoundMemberTy);
    assert(op->getOpcode() == BO_PtrMemD || op->getOpcode() == BO_PtrMemI);

    QualType fnType =
      op->getRHS()->getType()->castAs<MemberPointerType>()->getPointeeType();

    const FunctionProtoType *proto = fnType->castAs<FunctionProtoType>();
    QualType resultType = proto->getCallResultType(Context);
    ExprValueKind valueKind = Expr::getValueKindForType(proto->getReturnType());

    // Check that the object type isn't more qualified than the
    // member function we're calling.
    Qualifiers funcQuals = proto->getMethodQuals();

    QualType objectType = op->getLHS()->getType();
    if (op->getOpcode() == BO_PtrMemI)
      objectType = objectType->castAs<PointerType>()->getPointeeType();
    Qualifiers objectQuals = objectType.getQualifiers();

    Qualifiers difference = objectQuals - funcQuals;
    difference.removeObjCGCAttr();
    difference.removeAddressSpace();
    if (difference) {
      std::string qualsString = difference.getAsString();
      Diag(LParenLoc, diag::err_pointer_to_member_call_drops_quals)
        << fnType.getUnqualifiedType()
        << qualsString
        << (qualsString.find(' ') == std::string::npos ? 1 : 2);
    }

    CXXMemberCallExpr *call = CXXMemberCallExpr::Create(
        Context, MemExprE, Args, resultType, valueKind, RParenLoc,
        CurFPFeatureOverrides(), proto->getNumParams());

    if (CheckCallReturnType(proto->getReturnType(), op->getRHS()->getBeginLoc(),
                            call, nullptr))
      return ExprError();

    if (ConvertArgumentsForCall(call, op, nullptr, proto, Args, RParenLoc))
      return ExprError();

    if (CheckOtherCall(call, proto))
      return ExprError();

    return MaybeBindToTemporary(call);
  }

  // We only try to build a recovery expr at this level if we can preserve
  // the return type, otherwise we return ExprError() and let the caller
  // recover.
  auto BuildRecoveryExpr = [&](QualType Type) {
    if (!AllowRecovery)
      return ExprError();
    std::vector<Expr *> SubExprs = {MemExprE};
    llvm::append_range(SubExprs, Args);
    return CreateRecoveryExpr(MemExprE->getBeginLoc(), RParenLoc, SubExprs,
                              Type);
  };
  if (isa<CXXPseudoDestructorExpr>(NakedMemExpr))
    return CallExpr::Create(Context, MemExprE, Args, Context.VoidTy, VK_PRValue,
                            RParenLoc, CurFPFeatureOverrides());

  UnbridgedCastsSet UnbridgedCasts;
  if (checkArgPlaceholdersForOverload(*this, Args, UnbridgedCasts))
    return ExprError();

  MemberExpr *MemExpr;
  CXXMethodDecl *Method = nullptr;
  DeclAccessPair FoundDecl = DeclAccessPair::make(nullptr, AS_public);
  NestedNameSpecifier *Qualifier = nullptr;
  if (isa<MemberExpr>(NakedMemExpr)) {
    MemExpr = cast<MemberExpr>(NakedMemExpr);
    Method = cast<CXXMethodDecl>(MemExpr->getMemberDecl());
    FoundDecl = MemExpr->getFoundDecl();
    Qualifier = MemExpr->getQualifier();
    UnbridgedCasts.restore();
  } else {
    UnresolvedMemberExpr *UnresExpr = cast<UnresolvedMemberExpr>(NakedMemExpr);
    Qualifier = UnresExpr->getQualifier();

    QualType ObjectType = UnresExpr->getBaseType();
    Expr::Classification ObjectClassification
      = UnresExpr->isArrow()? Expr::Classification::makeSimpleLValue()
                            : UnresExpr->getBase()->Classify(Context);

    // Add overload candidates
    OverloadCandidateSet CandidateSet(UnresExpr->getMemberLoc(),
                                      OverloadCandidateSet::CSK_Normal);

    // FIXME: avoid copy.
    TemplateArgumentListInfo TemplateArgsBuffer, *TemplateArgs = nullptr;
    if (UnresExpr->hasExplicitTemplateArgs()) {
      UnresExpr->copyTemplateArgumentsInto(TemplateArgsBuffer);
      TemplateArgs = &TemplateArgsBuffer;
    }

    for (UnresolvedMemberExpr::decls_iterator I = UnresExpr->decls_begin(),
           E = UnresExpr->decls_end(); I != E; ++I) {

      NamedDecl *Func = *I;
      CXXRecordDecl *ActingDC = cast<CXXRecordDecl>(Func->getDeclContext());
      if (isa<UsingShadowDecl>(Func))
        Func = cast<UsingShadowDecl>(Func)->getTargetDecl();


      // Microsoft supports direct constructor calls.
      if (getLangOpts().MicrosoftExt && isa<CXXConstructorDecl>(Func)) {
        AddOverloadCandidate(cast<CXXConstructorDecl>(Func), I.getPair(), Args,
                             CandidateSet,
                             /*SuppressUserConversions*/ false);
      } else if ((Method = dyn_cast<CXXMethodDecl>(Func))) {
        // If explicit template arguments were provided, we can't call a
        // non-template member function.
        if (TemplateArgs)
          continue;

        AddMethodCandidate(Method, I.getPair(), ActingDC, ObjectType,
                           ObjectClassification, Args, CandidateSet,
                           /*SuppressUserConversions=*/false);
      } else {
        AddMethodTemplateCandidate(
            cast<FunctionTemplateDecl>(Func), I.getPair(), ActingDC,
            TemplateArgs, ObjectType, ObjectClassification, Args, CandidateSet,
            /*SuppressUserConversions=*/false);
      }
    }

    DeclarationName DeclName = UnresExpr->getMemberName();

    UnbridgedCasts.restore();

    OverloadCandidateSet::iterator Best;
    bool Succeeded = false;
    switch (CandidateSet.BestViableFunction(*this, UnresExpr->getBeginLoc(),
                                            Best)) {
    case OR_Success:
      Method = cast<CXXMethodDecl>(Best->Function);
      FoundDecl = Best->FoundDecl;
      CheckUnresolvedMemberAccess(UnresExpr, Best->FoundDecl);
      if (DiagnoseUseOfOverloadedDecl(Best->FoundDecl, UnresExpr->getNameLoc()))
        break;
      // If FoundDecl is different from Method (such as if one is a template
      // and the other a specialization), make sure DiagnoseUseOfDecl is
      // called on both.
      // FIXME: This would be more comprehensively addressed by modifying
      // DiagnoseUseOfDecl to accept both the FoundDecl and the decl
      // being used.
      if (Method != FoundDecl.getDecl() &&
          DiagnoseUseOfOverloadedDecl(Method, UnresExpr->getNameLoc()))
        break;
      Succeeded = true;
      break;

    case OR_No_Viable_Function:
      CandidateSet.NoteCandidates(
          PartialDiagnosticAt(
              UnresExpr->getMemberLoc(),
              PDiag(diag::err_ovl_no_viable_member_function_in_call)
                  << DeclName << MemExprE->getSourceRange()),
          *this, OCD_AllCandidates, Args);
      break;
    case OR_Ambiguous:
      CandidateSet.NoteCandidates(
          PartialDiagnosticAt(UnresExpr->getMemberLoc(),
                              PDiag(diag::err_ovl_ambiguous_member_call)
                                  << DeclName << MemExprE->getSourceRange()),
          *this, OCD_AmbiguousCandidates, Args);
      break;
    case OR_Deleted:
      CandidateSet.NoteCandidates(
          PartialDiagnosticAt(UnresExpr->getMemberLoc(),
                              PDiag(diag::err_ovl_deleted_member_call)
                                  << DeclName << MemExprE->getSourceRange()),
          *this, OCD_AllCandidates, Args);
      break;
    }
    // Overload resolution fails, try to recover.
    if (!Succeeded)
      return BuildRecoveryExpr(chooseRecoveryType(CandidateSet, &Best));

    MemExprE = FixOverloadedFunctionReference(MemExprE, FoundDecl, Method);

    // If overload resolution picked a static member, build a
    // non-member call based on that function.
    if (Method->isStatic()) {
      return BuildResolvedCallExpr(MemExprE, Method, LParenLoc, Args, RParenLoc,
                                   ExecConfig, IsExecConfig);
    }

    MemExpr = cast<MemberExpr>(MemExprE->IgnoreParens());
  }

  QualType ResultType = Method->getReturnType();
  ExprValueKind VK = Expr::getValueKindForType(ResultType);
  ResultType = ResultType.getNonLValueExprType(Context);

  assert(Method && "Member call to something that isn't a method?");
  const auto *Proto = Method->getType()->castAs<FunctionProtoType>();
  CXXMemberCallExpr *TheCall = CXXMemberCallExpr::Create(
      Context, MemExprE, Args, ResultType, VK, RParenLoc,
      CurFPFeatureOverrides(), Proto->getNumParams());

  // Check for a valid return type.
  if (CheckCallReturnType(Method->getReturnType(), MemExpr->getMemberLoc(),
                          TheCall, Method))
    return BuildRecoveryExpr(ResultType);

  // Convert the object argument (for a non-static member function call).
  // We only need to do this if there was actually an overload; otherwise
  // it was done at lookup.
  if (!Method->isStatic()) {
    ExprResult ObjectArg =
      PerformObjectArgumentInitialization(MemExpr->getBase(), Qualifier,
                                          FoundDecl, Method);
    if (ObjectArg.isInvalid())
      return ExprError();
    MemExpr->setBase(ObjectArg.get());
  }

  // Convert the rest of the arguments
  if (ConvertArgumentsForCall(TheCall, MemExpr, Method, Proto, Args,
                              RParenLoc))
    return BuildRecoveryExpr(ResultType);

  DiagnoseSentinelCalls(Method, LParenLoc, Args);

  if (CheckFunctionCall(Method, TheCall, Proto))
    return ExprError();

  // In the case the method to call was not selected by the overloading
  // resolution process, we still need to handle the enable_if attribute. Do
  // that here, so it will not hide previous -- and more relevant -- errors.
  if (auto *MemE = dyn_cast<MemberExpr>(NakedMemExpr)) {
    if (const EnableIfAttr *Attr =
            CheckEnableIf(Method, LParenLoc, Args, true)) {
      Diag(MemE->getMemberLoc(),
           diag::err_ovl_no_viable_member_function_in_call)
          << Method << Method->getSourceRange();
      Diag(Method->getLocation(),
           diag::note_ovl_candidate_disabled_by_function_cond_attr)
          << Attr->getCond()->getSourceRange() << Attr->getMessage();
      return ExprError();
    }
  }

  if ((isa<CXXConstructorDecl>(CurContext) ||
       isa<CXXDestructorDecl>(CurContext)) &&
      TheCall->getMethodDecl()->isPure()) {
    const CXXMethodDecl *MD = TheCall->getMethodDecl();

    if (isa<CXXThisExpr>(MemExpr->getBase()->IgnoreParenCasts()) &&
        MemExpr->performsVirtualDispatch(getLangOpts())) {
      Diag(MemExpr->getBeginLoc(),
           diag::warn_call_to_pure_virtual_member_function_from_ctor_dtor)
          << MD->getDeclName() << isa<CXXDestructorDecl>(CurContext)
          << MD->getParent();

      Diag(MD->getBeginLoc(), diag::note_previous_decl) << MD->getDeclName();
      if (getLangOpts().AppleKext)
        Diag(MemExpr->getBeginLoc(), diag::note_pure_qualified_call_kext)
            << MD->getParent() << MD->getDeclName();
    }
  }

  if (CXXDestructorDecl *DD =
          dyn_cast<CXXDestructorDecl>(TheCall->getMethodDecl())) {
    // a->A::f() doesn't go through the vtable, except in AppleKext mode.
    bool CallCanBeVirtual = !MemExpr->hasQualifier() || getLangOpts().AppleKext;
    CheckVirtualDtorCall(DD, MemExpr->getBeginLoc(), /*IsDelete=*/false,
                         CallCanBeVirtual, /*WarnOnNonAbstractTypes=*/true,
                         MemExpr->getMemberLoc());
  }

  return CheckForImmediateInvocation(MaybeBindToTemporary(TheCall),
                                     TheCall->getMethodDecl());
}

/// BuildCallToObjectOfClassType - Build a call to an object of class
/// type (C++ [over.call.object]), which can end up invoking an
/// overloaded function call operator (@c operator()) or performing a
/// user-defined conversion on the object argument.
ExprResult
Sema::BuildCallToObjectOfClassType(Scope *S, Expr *Obj,
                                   SourceLocation LParenLoc,
                                   MultiExprArg Args,
                                   SourceLocation RParenLoc) {
  if (checkPlaceholderForOverload(*this, Obj))
    return ExprError();
  ExprResult Object = Obj;

  UnbridgedCastsSet UnbridgedCasts;
  if (checkArgPlaceholdersForOverload(*this, Args, UnbridgedCasts))
    return ExprError();

  assert(Object.get()->getType()->isRecordType() &&
         "Requires object type argument");

  // C++ [over.call.object]p1:
  //  If the primary-expression E in the function call syntax
  //  evaluates to a class object of type "cv T", then the set of
  //  candidate functions includes at least the function call
  //  operators of T. The function call operators of T are obtained by
  //  ordinary lookup of the name operator() in the context of
  //  (E).operator().
  OverloadCandidateSet CandidateSet(LParenLoc,
                                    OverloadCandidateSet::CSK_Operator);
  DeclarationName OpName = Context.DeclarationNames.getCXXOperatorName(OO_Call);

  if (RequireCompleteType(LParenLoc, Object.get()->getType(),
                          diag::err_incomplete_object_call, Object.get()))
    return true;

  const auto *Record = Object.get()->getType()->castAs<RecordType>();
  LookupResult R(*this, OpName, LParenLoc, LookupOrdinaryName);
  LookupQualifiedName(R, Record->getDecl());
  R.suppressDiagnostics();

  for (LookupResult::iterator Oper = R.begin(), OperEnd = R.end();
       Oper != OperEnd; ++Oper) {
    AddMethodCandidate(Oper.getPair(), Object.get()->getType(),
                       Object.get()->Classify(Context), Args, CandidateSet,
                       /*SuppressUserConversion=*/false);
  }

  // C++ [over.call.object]p2:
  //   In addition, for each (non-explicit in C++0x) conversion function
  //   declared in T of the form
  //
  //        operator conversion-type-id () cv-qualifier;
  //
  //   where cv-qualifier is the same cv-qualification as, or a
  //   greater cv-qualification than, cv, and where conversion-type-id
  //   denotes the type "pointer to function of (P1,...,Pn) returning
  //   R", or the type "reference to pointer to function of
  //   (P1,...,Pn) returning R", or the type "reference to function
  //   of (P1,...,Pn) returning R", a surrogate call function [...]
  //   is also considered as a candidate function. Similarly,
  //   surrogate call functions are added to the set of candidate
  //   functions for each conversion function declared in an
  //   accessible base class provided the function is not hidden
  //   within T by another intervening declaration.
  const auto &Conversions =
      cast<CXXRecordDecl>(Record->getDecl())->getVisibleConversionFunctions();
  for (auto I = Conversions.begin(), E = Conversions.end(); I != E; ++I) {
    NamedDecl *D = *I;
    CXXRecordDecl *ActingContext = cast<CXXRecordDecl>(D->getDeclContext());
    if (isa<UsingShadowDecl>(D))
      D = cast<UsingShadowDecl>(D)->getTargetDecl();

    // Skip over templated conversion functions; they aren't
    // surrogates.
    if (isa<FunctionTemplateDecl>(D))
      continue;

    CXXConversionDecl *Conv = cast<CXXConversionDecl>(D);
    if (!Conv->isExplicit()) {
      // Strip the reference type (if any) and then the pointer type (if
      // any) to get down to what might be a function type.
      QualType ConvType = Conv->getConversionType().getNonReferenceType();
      if (const PointerType *ConvPtrType = ConvType->getAs<PointerType>())
        ConvType = ConvPtrType->getPointeeType();

      if (const FunctionProtoType *Proto = ConvType->getAs<FunctionProtoType>())
      {
        AddSurrogateCandidate(Conv, I.getPair(), ActingContext, Proto,
                              Object.get(), Args, CandidateSet);
      }
    }
  }

  bool HadMultipleCandidates = (CandidateSet.size() > 1);

  // Perform overload resolution.
  OverloadCandidateSet::iterator Best;
  switch (CandidateSet.BestViableFunction(*this, Object.get()->getBeginLoc(),
                                          Best)) {
  case OR_Success:
    // Overload resolution succeeded; we'll build the appropriate call
    // below.
    break;

  case OR_No_Viable_Function: {
    PartialDiagnostic PD =
        CandidateSet.empty()
            ? (PDiag(diag::err_ovl_no_oper)
               << Object.get()->getType() << /*call*/ 1
               << Object.get()->getSourceRange())
            : (PDiag(diag::err_ovl_no_viable_object_call)
               << Object.get()->getType() << Object.get()->getSourceRange());
    CandidateSet.NoteCandidates(
        PartialDiagnosticAt(Object.get()->getBeginLoc(), PD), *this,
        OCD_AllCandidates, Args);
    break;
  }
  case OR_Ambiguous:
    CandidateSet.NoteCandidates(
        PartialDiagnosticAt(Object.get()->getBeginLoc(),
                            PDiag(diag::err_ovl_ambiguous_object_call)
                                << Object.get()->getType()
                                << Object.get()->getSourceRange()),
        *this, OCD_AmbiguousCandidates, Args);
    break;

  case OR_Deleted:
    CandidateSet.NoteCandidates(
        PartialDiagnosticAt(Object.get()->getBeginLoc(),
                            PDiag(diag::err_ovl_deleted_object_call)
                                << Object.get()->getType()
                                << Object.get()->getSourceRange()),
        *this, OCD_AllCandidates, Args);
    break;
  }

  if (Best == CandidateSet.end())
    return true;

  UnbridgedCasts.restore();

  if (Best->Function == nullptr) {
    // Since there is no function declaration, this is one of the
    // surrogate candidates. Dig out the conversion function.
    CXXConversionDecl *Conv
      = cast<CXXConversionDecl>(
                         Best->Conversions[0].UserDefined.ConversionFunction);

    CheckMemberOperatorAccess(LParenLoc, Object.get(), nullptr,
                              Best->FoundDecl);
    if (DiagnoseUseOfDecl(Best->FoundDecl, LParenLoc))
      return ExprError();
    assert(Conv == Best->FoundDecl.getDecl() &&
             "Found Decl & conversion-to-functionptr should be same, right?!");
    // We selected one of the surrogate functions that converts the
    // object parameter to a function pointer. Perform the conversion
    // on the object argument, then let BuildCallExpr finish the job.

    // Create an implicit member expr to refer to the conversion operator.
    // and then call it.
    ExprResult Call = BuildCXXMemberCallExpr(Object.get(), Best->FoundDecl,
                                             Conv, HadMultipleCandidates);
    if (Call.isInvalid())
      return ExprError();
    // Record usage of conversion in an implicit cast.
    Call = ImplicitCastExpr::Create(
        Context, Call.get()->getType(), CK_UserDefinedConversion, Call.get(),
        nullptr, VK_PRValue, CurFPFeatureOverrides());

    return BuildCallExpr(S, Call.get(), LParenLoc, Args, RParenLoc);
  }

  CheckMemberOperatorAccess(LParenLoc, Object.get(), nullptr, Best->FoundDecl);

  // We found an overloaded operator(). Build a CXXOperatorCallExpr
  // that calls this method, using Object for the implicit object
  // parameter and passing along the remaining arguments.
  CXXMethodDecl *Method = cast<CXXMethodDecl>(Best->Function);

  // An error diagnostic has already been printed when parsing the declaration.
  if (Method->isInvalidDecl())
    return ExprError();

  const auto *Proto = Method->getType()->castAs<FunctionProtoType>();
  unsigned NumParams = Proto->getNumParams();

  DeclarationNameInfo OpLocInfo(
               Context.DeclarationNames.getCXXOperatorName(OO_Call), LParenLoc);
  OpLocInfo.setCXXOperatorNameRange(SourceRange(LParenLoc, RParenLoc));
  ExprResult NewFn = CreateFunctionRefExpr(*this, Method, Best->FoundDecl,
                                           Obj, HadMultipleCandidates,
                                           OpLocInfo.getLoc(),
                                           OpLocInfo.getInfo());
  if (NewFn.isInvalid())
    return true;

  SmallVector<Expr *, 8> MethodArgs;
  MethodArgs.reserve(NumParams + 1);

  bool IsError = false;

  // Initialize the implicit object parameter if needed.
  // Since C++23, this could also be a call to a static call operator
  // which we emit as a regular CallExpr.
  if (Method->isInstance()) {
    ExprResult ObjRes = PerformObjectArgumentInitialization(
        Object.get(), /*Qualifier=*/nullptr, Best->FoundDecl, Method);
    if (ObjRes.isInvalid())
      IsError = true;
    else
      Object = ObjRes;
    MethodArgs.push_back(Object.get());
  }

  IsError |= PrepareArgumentsForCallToObjectOfClassType(
      *this, MethodArgs, Method, Args, LParenLoc);

  // If this is a variadic call, handle args passed through "...".
  if (Proto->isVariadic()) {
    // Promote the arguments (C99 6.5.2.2p7).
    for (unsigned i = NumParams, e = Args.size(); i < e; i++) {
      ExprResult Arg = DefaultVariadicArgumentPromotion(Args[i], VariadicMethod,
                                                        nullptr);
      IsError |= Arg.isInvalid();
      MethodArgs.push_back(Arg.get());
    }
  }

  if (IsError)
    return true;

  DiagnoseSentinelCalls(Method, LParenLoc, Args);

  // Once we've built TheCall, all of the expressions are properly owned.
  QualType ResultTy = Method->getReturnType();
  ExprValueKind VK = Expr::getValueKindForType(ResultTy);
  ResultTy = ResultTy.getNonLValueExprType(Context);

  CallExpr *TheCall;
  if (Method->isInstance())
    TheCall = CXXOperatorCallExpr::Create(Context, OO_Call, NewFn.get(),
                                          MethodArgs, ResultTy, VK, RParenLoc,
                                          CurFPFeatureOverrides());
  else
    TheCall = CallExpr::Create(Context, NewFn.get(), MethodArgs, ResultTy, VK,
                               RParenLoc, CurFPFeatureOverrides());

  if (CheckCallReturnType(Method->getReturnType(), LParenLoc, TheCall, Method))
    return true;

  if (CheckFunctionCall(Method, TheCall, Proto))
    return true;

  return CheckForImmediateInvocation(MaybeBindToTemporary(TheCall), Method);
}

/// BuildOverloadedArrowExpr - Build a call to an overloaded @c operator->
///  (if one exists), where @c Base is an expression of class type and
/// @c Member is the name of the member we're trying to find.
ExprResult
Sema::BuildOverloadedArrowExpr(Scope *S, Expr *Base, SourceLocation OpLoc,
                               bool *NoArrowOperatorFound) {
  assert(Base->getType()->isRecordType() &&
         "left-hand side must have class type");

  if (checkPlaceholderForOverload(*this, Base))
    return ExprError();

  SourceLocation Loc = Base->getExprLoc();

  // C++ [over.ref]p1:
  //
  //   [...] An expression x->m is interpreted as (x.operator->())->m
  //   for a class object x of type T if T::operator->() exists and if
  //   the operator is selected as the best match function by the
  //   overload resolution mechanism (13.3).
  DeclarationName OpName =
    Context.DeclarationNames.getCXXOperatorName(OO_Arrow);
  OverloadCandidateSet CandidateSet(Loc, OverloadCandidateSet::CSK_Operator);

  if (RequireCompleteType(Loc, Base->getType(),
                          diag::err_typecheck_incomplete_tag, Base))
    return ExprError();

  LookupResult R(*this, OpName, OpLoc, LookupOrdinaryName);
  LookupQualifiedName(R, Base->getType()->castAs<RecordType>()->getDecl());
  R.suppressDiagnostics();

  for (LookupResult::iterator Oper = R.begin(), OperEnd = R.end();
       Oper != OperEnd; ++Oper) {
    AddMethodCandidate(Oper.getPair(), Base->getType(), Base->Classify(Context),
                       std::nullopt, CandidateSet,
                       /*SuppressUserConversion=*/false);
  }

  bool HadMultipleCandidates = (CandidateSet.size() > 1);

  // Perform overload resolution.
  OverloadCandidateSet::iterator Best;
  switch (CandidateSet.BestViableFunction(*this, OpLoc, Best)) {
  case OR_Success:
    // Overload resolution succeeded; we'll build the call below.
    break;

  case OR_No_Viable_Function: {
    auto Cands = CandidateSet.CompleteCandidates(*this, OCD_AllCandidates, Base);
    if (CandidateSet.empty()) {
      QualType BaseType = Base->getType();
      if (NoArrowOperatorFound) {
        // Report this specific error to the caller instead of emitting a
        // diagnostic, as requested.
        *NoArrowOperatorFound = true;
        return ExprError();
      }
      Diag(OpLoc, diag::err_typecheck_member_reference_arrow)
        << BaseType << Base->getSourceRange();
      if (BaseType->isRecordType() && !BaseType->isPointerType()) {
        Diag(OpLoc, diag::note_typecheck_member_reference_suggestion)
          << FixItHint::CreateReplacement(OpLoc, ".");
      }
    } else
      Diag(OpLoc, diag::err_ovl_no_viable_oper)
        << "operator->" << Base->getSourceRange();
    CandidateSet.NoteCandidates(*this, Base, Cands);
    return ExprError();
  }
  case OR_Ambiguous:
    CandidateSet.NoteCandidates(
        PartialDiagnosticAt(OpLoc, PDiag(diag::err_ovl_ambiguous_oper_unary)
                                       << "->" << Base->getType()
                                       << Base->getSourceRange()),
        *this, OCD_AmbiguousCandidates, Base);
    return ExprError();

  case OR_Deleted:
    CandidateSet.NoteCandidates(
        PartialDiagnosticAt(OpLoc, PDiag(diag::err_ovl_deleted_oper)
                                       << "->" << Base->getSourceRange()),
        *this, OCD_AllCandidates, Base);
    return ExprError();
  }

  CheckMemberOperatorAccess(OpLoc, Base, nullptr, Best->FoundDecl);

  // Convert the object parameter.
  CXXMethodDecl *Method = cast<CXXMethodDecl>(Best->Function);
  ExprResult BaseResult =
    PerformObjectArgumentInitialization(Base, /*Qualifier=*/nullptr,
                                        Best->FoundDecl, Method);
  if (BaseResult.isInvalid())
    return ExprError();
  Base = BaseResult.get();

  // Build the operator call.
  ExprResult FnExpr = CreateFunctionRefExpr(*this, Method, Best->FoundDecl,
                                            Base, HadMultipleCandidates, OpLoc);
  if (FnExpr.isInvalid())
    return ExprError();

  QualType ResultTy = Method->getReturnType();
  ExprValueKind VK = Expr::getValueKindForType(ResultTy);
  ResultTy = ResultTy.getNonLValueExprType(Context);
  CXXOperatorCallExpr *TheCall =
      CXXOperatorCallExpr::Create(Context, OO_Arrow, FnExpr.get(), Base,
                                  ResultTy, VK, OpLoc, CurFPFeatureOverrides());

  if (CheckCallReturnType(Method->getReturnType(), OpLoc, TheCall, Method))
    return ExprError();

  if (CheckFunctionCall(Method, TheCall,
                        Method->getType()->castAs<FunctionProtoType>()))
    return ExprError();

  return CheckForImmediateInvocation(MaybeBindToTemporary(TheCall), Method);
}

/// BuildLiteralOperatorCall - Build a UserDefinedLiteral by creating a call to
/// a literal operator described by the provided lookup results.
ExprResult Sema::BuildLiteralOperatorCall(LookupResult &R,
                                          DeclarationNameInfo &SuffixInfo,
                                          ArrayRef<Expr*> Args,
                                          SourceLocation LitEndLoc,
                                       TemplateArgumentListInfo *TemplateArgs) {
  SourceLocation UDSuffixLoc = SuffixInfo.getCXXLiteralOperatorNameLoc();

  OverloadCandidateSet CandidateSet(UDSuffixLoc,
                                    OverloadCandidateSet::CSK_Normal);
  AddNonMemberOperatorCandidates(R.asUnresolvedSet(), Args, CandidateSet,
                                 TemplateArgs);

  bool HadMultipleCandidates = (CandidateSet.size() > 1);

  // Perform overload resolution. This will usually be trivial, but might need
  // to perform substitutions for a literal operator template.
  OverloadCandidateSet::iterator Best;
  switch (CandidateSet.BestViableFunction(*this, UDSuffixLoc, Best)) {
  case OR_Success:
  case OR_Deleted:
    break;

  case OR_No_Viable_Function:
    CandidateSet.NoteCandidates(
        PartialDiagnosticAt(UDSuffixLoc,
                            PDiag(diag::err_ovl_no_viable_function_in_call)
                                << R.getLookupName()),
        *this, OCD_AllCandidates, Args);
    return ExprError();

  case OR_Ambiguous:
    CandidateSet.NoteCandidates(
        PartialDiagnosticAt(R.getNameLoc(), PDiag(diag::err_ovl_ambiguous_call)
                                                << R.getLookupName()),
        *this, OCD_AmbiguousCandidates, Args);
    return ExprError();
  }

  FunctionDecl *FD = Best->Function;
  ExprResult Fn = CreateFunctionRefExpr(*this, FD, Best->FoundDecl,
                                        nullptr, HadMultipleCandidates,
                                        SuffixInfo.getLoc(),
                                        SuffixInfo.getInfo());
  if (Fn.isInvalid())
    return true;

  // Check the argument types. This should almost always be a no-op, except
  // that array-to-pointer decay is applied to string literals.
  Expr *ConvArgs[2];
  for (unsigned ArgIdx = 0, N = Args.size(); ArgIdx != N; ++ArgIdx) {
    ExprResult InputInit = PerformCopyInitialization(
      InitializedEntity::InitializeParameter(Context, FD->getParamDecl(ArgIdx)),
      SourceLocation(), Args[ArgIdx]);
    if (InputInit.isInvalid())
      return true;
    ConvArgs[ArgIdx] = InputInit.get();
  }

  QualType ResultTy = FD->getReturnType();
  ExprValueKind VK = Expr::getValueKindForType(ResultTy);
  ResultTy = ResultTy.getNonLValueExprType(Context);

  UserDefinedLiteral *UDL = UserDefinedLiteral::Create(
      Context, Fn.get(), llvm::ArrayRef(ConvArgs, Args.size()), ResultTy, VK,
      LitEndLoc, UDSuffixLoc, CurFPFeatureOverrides());

  if (CheckCallReturnType(FD->getReturnType(), UDSuffixLoc, UDL, FD))
    return ExprError();

  if (CheckFunctionCall(FD, UDL, nullptr))
    return ExprError();

  return CheckForImmediateInvocation(MaybeBindToTemporary(UDL), FD);
}

/// Build a call to 'begin' or 'end' for a C++11 for-range statement. If the
/// given LookupResult is non-empty, it is assumed to describe a member which
/// will be invoked. Otherwise, the function will be found via argument
/// dependent lookup.
/// CallExpr is set to a valid expression and FRS_Success returned on success,
/// otherwise CallExpr is set to ExprError() and some non-success value
/// is returned.
Sema::ForRangeStatus
Sema::BuildForRangeBeginEndCall(SourceLocation Loc,
                                SourceLocation RangeLoc,
                                const DeclarationNameInfo &NameInfo,
                                LookupResult &MemberLookup,
                                OverloadCandidateSet *CandidateSet,
                                Expr *Range, ExprResult *CallExpr) {
  Scope *S = nullptr;

  CandidateSet->clear(OverloadCandidateSet::CSK_Normal);
  if (!MemberLookup.empty()) {
    ExprResult MemberRef =
        BuildMemberReferenceExpr(Range, Range->getType(), Loc,
                                 /*IsPtr=*/false, CXXScopeSpec(),
                                 /*TemplateKWLoc=*/SourceLocation(),
                                 /*FirstQualifierInScope=*/nullptr,
                                 MemberLookup,
                                 /*TemplateArgs=*/nullptr, S);
    if (MemberRef.isInvalid()) {
      *CallExpr = ExprError();
      return FRS_DiagnosticIssued;
    }
    *CallExpr =
        BuildCallExpr(S, MemberRef.get(), Loc, std::nullopt, Loc, nullptr);
    if (CallExpr->isInvalid()) {
      *CallExpr = ExprError();
      return FRS_DiagnosticIssued;
    }
  } else {
    ExprResult FnR = CreateUnresolvedLookupExpr(/*NamingClass=*/nullptr,
                                                NestedNameSpecifierLoc(),
                                                NameInfo, UnresolvedSet<0>());
    if (FnR.isInvalid())
      return FRS_DiagnosticIssued;
    UnresolvedLookupExpr *Fn = cast<UnresolvedLookupExpr>(FnR.get());

    bool CandidateSetError = buildOverloadedCallSet(S, Fn, Fn, Range, Loc,
                                                    CandidateSet, CallExpr);
    if (CandidateSet->empty() || CandidateSetError) {
      *CallExpr = ExprError();
      return FRS_NoViableFunction;
    }
    OverloadCandidateSet::iterator Best;
    OverloadingResult OverloadResult =
        CandidateSet->BestViableFunction(*this, Fn->getBeginLoc(), Best);

    if (OverloadResult == OR_No_Viable_Function) {
      *CallExpr = ExprError();
      return FRS_NoViableFunction;
    }
    *CallExpr = FinishOverloadedCallExpr(*this, S, Fn, Fn, Loc, Range,
                                         Loc, nullptr, CandidateSet, &Best,
                                         OverloadResult,
                                         /*AllowTypoCorrection=*/false);
    if (CallExpr->isInvalid() || OverloadResult != OR_Success) {
      *CallExpr = ExprError();
      return FRS_DiagnosticIssued;
    }
  }
  return FRS_Success;
}


/// FixOverloadedFunctionReference - E is an expression that refers to
/// a C++ overloaded function (possibly with some parentheses and
/// perhaps a '&' around it). We have resolved the overloaded function
/// to the function declaration Fn, so patch up the expression E to
/// refer (possibly indirectly) to Fn. Returns the new expr.
Expr *Sema::FixOverloadedFunctionReference(Expr *E, DeclAccessPair Found,
                                           FunctionDecl *Fn) {
  if (ParenExpr *PE = dyn_cast<ParenExpr>(E)) {
    Expr *SubExpr = FixOverloadedFunctionReference(PE->getSubExpr(),
                                                   Found, Fn);
    if (SubExpr == PE->getSubExpr())
      return PE;

    return new (Context) ParenExpr(PE->getLParen(), PE->getRParen(), SubExpr);
  }

  if (ImplicitCastExpr *ICE = dyn_cast<ImplicitCastExpr>(E)) {
    Expr *SubExpr = FixOverloadedFunctionReference(ICE->getSubExpr(),
                                                   Found, Fn);
    assert(Context.hasSameType(ICE->getSubExpr()->getType(),
                               SubExpr->getType()) &&
           "Implicit cast type cannot be determined from overload");
    assert(ICE->path_empty() && "fixing up hierarchy conversion?");
    if (SubExpr == ICE->getSubExpr())
      return ICE;

    return ImplicitCastExpr::Create(Context, ICE->getType(), ICE->getCastKind(),
                                    SubExpr, nullptr, ICE->getValueKind(),
                                    CurFPFeatureOverrides());
  }

  if (auto *GSE = dyn_cast<GenericSelectionExpr>(E)) {
    if (!GSE->isResultDependent()) {
      Expr *SubExpr =
          FixOverloadedFunctionReference(GSE->getResultExpr(), Found, Fn);
      if (SubExpr == GSE->getResultExpr())
        return GSE;

      // Replace the resulting type information before rebuilding the generic
      // selection expression.
      ArrayRef<Expr *> A = GSE->getAssocExprs();
      SmallVector<Expr *, 4> AssocExprs(A.begin(), A.end());
      unsigned ResultIdx = GSE->getResultIndex();
      AssocExprs[ResultIdx] = SubExpr;

      return GenericSelectionExpr::Create(
          Context, GSE->getGenericLoc(), GSE->getControllingExpr(),
          GSE->getAssocTypeSourceInfos(), AssocExprs, GSE->getDefaultLoc(),
          GSE->getRParenLoc(), GSE->containsUnexpandedParameterPack(),
          ResultIdx);
    }
    // Rather than fall through to the unreachable, return the original generic
    // selection expression.
    return GSE;
  }

  if (UnaryOperator *UnOp = dyn_cast<UnaryOperator>(E)) {
    assert(UnOp->getOpcode() == UO_AddrOf &&
           "Can only take the address of an overloaded function");
    if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn)) {
      if (Method->isStatic()) {
        // Do nothing: static member functions aren't any different
        // from non-member functions.
      } else {
        // Fix the subexpression, which really has to be an
        // UnresolvedLookupExpr holding an overloaded member function
        // or template.
        Expr *SubExpr = FixOverloadedFunctionReference(UnOp->getSubExpr(),
                                                       Found, Fn);
        if (SubExpr == UnOp->getSubExpr())
          return UnOp;

        assert(isa<DeclRefExpr>(SubExpr)
               && "fixed to something other than a decl ref");
        assert(cast<DeclRefExpr>(SubExpr)->getQualifier()
               && "fixed to a member ref with no nested name qualifier");

        // We have taken the address of a pointer to member
        // function. Perform the computation here so that we get the
        // appropriate pointer to member type.
        QualType ClassType
          = Context.getTypeDeclType(cast<RecordDecl>(Method->getDeclContext()));
        QualType MemPtrType
          = Context.getMemberPointerType(Fn->getType(), ClassType.getTypePtr());
        // Under the MS ABI, lock down the inheritance model now.
        if (Context.getTargetInfo().getCXXABI().isMicrosoft())
          (void)isCompleteType(UnOp->getOperatorLoc(), MemPtrType);

        return UnaryOperator::Create(
            Context, SubExpr, UO_AddrOf, MemPtrType, VK_PRValue, OK_Ordinary,
            UnOp->getOperatorLoc(), false, CurFPFeatureOverrides());
      }
    }
    Expr *SubExpr = FixOverloadedFunctionReference(UnOp->getSubExpr(),
                                                   Found, Fn);
    if (SubExpr == UnOp->getSubExpr())
      return UnOp;

    // FIXME: This can't currently fail, but in principle it could.
    return CreateBuiltinUnaryOp(UnOp->getOperatorLoc(), UO_AddrOf, SubExpr)
        .get();
  }

  if (UnresolvedLookupExpr *ULE = dyn_cast<UnresolvedLookupExpr>(E)) {
    // FIXME: avoid copy.
    TemplateArgumentListInfo TemplateArgsBuffer, *TemplateArgs = nullptr;
    if (ULE->hasExplicitTemplateArgs()) {
      ULE->copyTemplateArgumentsInto(TemplateArgsBuffer);
      TemplateArgs = &TemplateArgsBuffer;
    }

    QualType Type = Fn->getType();
    ExprValueKind ValueKind = getLangOpts().CPlusPlus ? VK_LValue : VK_PRValue;

    // FIXME: Duplicated from BuildDeclarationNameExpr.
    if (unsigned BID = Fn->getBuiltinID()) {
      if (!Context.BuiltinInfo.isDirectlyAddressable(BID)) {
        Type = Context.BuiltinFnTy;
        ValueKind = VK_PRValue;
      }
    }

    DeclRefExpr *DRE = BuildDeclRefExpr(
        Fn, Type, ValueKind, ULE->getNameInfo(), ULE->getQualifierLoc(),
        Found.getDecl(), ULE->getTemplateKeywordLoc(), TemplateArgs);
    DRE->setHadMultipleCandidates(ULE->getNumDecls() > 1);
    return DRE;
  }

  if (UnresolvedMemberExpr *MemExpr = dyn_cast<UnresolvedMemberExpr>(E)) {
    // FIXME: avoid copy.
    TemplateArgumentListInfo TemplateArgsBuffer, *TemplateArgs = nullptr;
    if (MemExpr->hasExplicitTemplateArgs()) {
      MemExpr->copyTemplateArgumentsInto(TemplateArgsBuffer);
      TemplateArgs = &TemplateArgsBuffer;
    }

    Expr *Base;

    // If we're filling in a static method where we used to have an
    // implicit member access, rewrite to a simple decl ref.
    if (MemExpr->isImplicitAccess()) {
      if (cast<CXXMethodDecl>(Fn)->isStatic()) {
        DeclRefExpr *DRE = BuildDeclRefExpr(
            Fn, Fn->getType(), VK_LValue, MemExpr->getNameInfo(),
            MemExpr->getQualifierLoc(), Found.getDecl(),
            MemExpr->getTemplateKeywordLoc(), TemplateArgs);
        DRE->setHadMultipleCandidates(MemExpr->getNumDecls() > 1);
        return DRE;
      } else {
        SourceLocation Loc = MemExpr->getMemberLoc();
        if (MemExpr->getQualifier())
          Loc = MemExpr->getQualifierLoc().getBeginLoc();
        Base =
            BuildCXXThisExpr(Loc, MemExpr->getBaseType(), /*IsImplicit=*/true);
      }
    } else
      Base = MemExpr->getBase();

    ExprValueKind valueKind;
    QualType type;
    if (cast<CXXMethodDecl>(Fn)->isStatic()) {
      valueKind = VK_LValue;
      type = Fn->getType();
    } else {
      valueKind = VK_PRValue;
      type = Context.BoundMemberTy;
    }

    return BuildMemberExpr(
        Base, MemExpr->isArrow(), MemExpr->getOperatorLoc(),
        MemExpr->getQualifierLoc(), MemExpr->getTemplateKeywordLoc(), Fn, Found,
        /*HadMultipleCandidates=*/true, MemExpr->getMemberNameInfo(),
        type, valueKind, OK_Ordinary, TemplateArgs);
  }

  llvm_unreachable("Invalid reference to overloaded function");
}

ExprResult Sema::FixOverloadedFunctionReference(ExprResult E,
                                                DeclAccessPair Found,
                                                FunctionDecl *Fn) {
  return FixOverloadedFunctionReference(E.get(), Found, Fn);
}

bool clang::shouldEnforceArgLimit(bool PartialOverloading,
                                  FunctionDecl *Function) {
  if (!PartialOverloading || !Function)
    return true;
  if (Function->isVariadic())
    return false;
  if (const auto *Proto =
          dyn_cast<FunctionProtoType>(Function->getFunctionType()))
    if (Proto->isTemplateVariadic())
      return false;
  if (auto *Pattern = Function->getTemplateInstantiationPattern())
    if (const auto *Proto =
            dyn_cast<FunctionProtoType>(Pattern->getFunctionType()))
      if (Proto->isTemplateVariadic())
        return false;
  return true;
}