diff --git a/clang/include/clang/Basic/Specifiers.h b/clang/include/clang/Basic/Specifiers.h
@@ -20,6 +20,14 @@
 #include "llvm/Support/ErrorHandling.h"
 
 namespace clang {
+
+  /// Define the meaning of possible values of the kind in ExplicitSpecifier.
+  enum class ExplicitSpecKind : unsigned {
+    ResolvedFalse,
+    ResolvedTrue,
+    Unresolved,
+  };
+
   /// Specifies the width of a type, e.g., short, long, or long long.
   enum TypeSpecifierWidth {
     TSW_unspecified,

diff --git a/clang/include/clang/Sema/DeclSpec.h b/clang/include/clang/Sema/DeclSpec.h
@@ -22,6 +22,7 @@
 #ifndef LLVM_CLANG_SEMA_DECLSPEC_H
 #define LLVM_CLANG_SEMA_DECLSPEC_H
 
+#include "clang/AST/DeclCXX.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/Basic/ExceptionSpecificationType.h"
 #include "clang/Basic/Lambda.h"
@@ -356,7 +357,6 @@ class DeclSpec {
   unsigned FS_inline_specified : 1;
   unsigned FS_forceinline_specified: 1;
   unsigned FS_virtual_specified : 1;
-  unsigned FS_explicit_specified : 1;
   unsigned FS_noreturn_specified : 1;
 
   // friend-specifier
@@ -371,6 +371,9 @@ class DeclSpec {
     Expr *ExprRep;
   };
 
+  /// ExplicitSpecifier - Store information about explicit spicifer.
+  ExplicitSpecifier FS_explicit_specifier;
+
   // attributes.
   ParsedAttributes Attrs;
 
@@ -393,6 +396,7 @@ class DeclSpec {
   SourceLocation TQ_constLoc, TQ_restrictLoc, TQ_volatileLoc, TQ_atomicLoc,
       TQ_unalignedLoc;
   SourceLocation FS_inlineLoc, FS_virtualLoc, FS_explicitLoc, FS_noreturnLoc;
+  SourceLocation FS_explicitCloseParenLoc;
   SourceLocation FS_forceinlineLoc;
   SourceLocation FriendLoc, ModulePrivateLoc, ConstexprLoc;
   SourceLocation TQ_pipeLoc;
@@ -420,31 +424,18 @@ class DeclSpec {
   }
 
   DeclSpec(AttributeFactory &attrFactory)
-    : StorageClassSpec(SCS_unspecified),
-      ThreadStorageClassSpec(TSCS_unspecified),
-      SCS_extern_in_linkage_spec(false),
-      TypeSpecWidth(TSW_unspecified),
-      TypeSpecComplex(TSC_unspecified),
-      TypeSpecSign(TSS_unspecified),
-      TypeSpecType(TST_unspecified),
-      TypeAltiVecVector(false),
-      TypeAltiVecPixel(false),
-      TypeAltiVecBool(false),
-      TypeSpecOwned(false),
-      TypeSpecPipe(false),
-      TypeSpecSat(false),
-      TypeQualifiers(TQ_unspecified),
-      FS_inline_specified(false),
-      FS_forceinline_specified(false),
-      FS_virtual_specified(false),
-      FS_explicit_specified(false),
-      FS_noreturn_specified(false),
-      Friend_specified(false),
-      Constexpr_specified(false),
-      Attrs(attrFactory),
-      writtenBS(),
-      ObjCQualifiers(nullptr) {
-  }
+      : StorageClassSpec(SCS_unspecified),
+        ThreadStorageClassSpec(TSCS_unspecified),
+        SCS_extern_in_linkage_spec(false), TypeSpecWidth(TSW_unspecified),
+        TypeSpecComplex(TSC_unspecified), TypeSpecSign(TSS_unspecified),
+        TypeSpecType(TST_unspecified), TypeAltiVecVector(false),
+        TypeAltiVecPixel(false), TypeAltiVecBool(false), TypeSpecOwned(false),
+        TypeSpecPipe(false), TypeSpecSat(false), TypeQualifiers(TQ_unspecified),
+        FS_inline_specified(false), FS_forceinline_specified(false),
+        FS_virtual_specified(false), FS_noreturn_specified(false),
+        Friend_specified(false), Constexpr_specified(false),
+        FS_explicit_specifier(), Attrs(attrFactory), writtenBS(),
+        ObjCQualifiers(nullptr) {}
 
   // storage-class-specifier
   SCS getStorageClassSpec() const { return (SCS)StorageClassSpec; }
@@ -570,11 +561,22 @@ class DeclSpec {
     return FS_inline_specified ? FS_inlineLoc : FS_forceinlineLoc;
   }
 
+  ExplicitSpecifier getExplicitSpecifier() const {
+    return FS_explicit_specifier;
+  }
+
   bool isVirtualSpecified() const { return FS_virtual_specified; }
   SourceLocation getVirtualSpecLoc() const { return FS_virtualLoc; }
 
-  bool isExplicitSpecified() const { return FS_explicit_specified; }
+  bool hasExplicitSpecifier() const {
+    return FS_explicit_specifier.isSpecified();
+  }
   SourceLocation getExplicitSpecLoc() const { return FS_explicitLoc; }
+  SourceRange getExplicitSpecRange() const {
+    return FS_explicit_specifier.getExpr()
+               ? SourceRange(FS_explicitLoc, FS_explicitCloseParenLoc)
+               : SourceRange(FS_explicitLoc);
+  }
 
   bool isNoreturnSpecified() const { return FS_noreturn_specified; }
   SourceLocation getNoreturnSpecLoc() const { return FS_noreturnLoc; }
@@ -586,8 +588,9 @@ class DeclSpec {
     FS_forceinlineLoc = SourceLocation();
     FS_virtual_specified = false;
     FS_virtualLoc = SourceLocation();
-    FS_explicit_specified = false;
+    FS_explicit_specifier = ExplicitSpecifier();
     FS_explicitLoc = SourceLocation();
+    FS_explicitCloseParenLoc = SourceLocation();
     FS_noreturn_specified = false;
     FS_noreturnLoc = SourceLocation();
   }
@@ -706,7 +709,8 @@ class DeclSpec {
   bool setFunctionSpecVirtual(SourceLocation Loc, const char *&PrevSpec,
                               unsigned &DiagID);
   bool setFunctionSpecExplicit(SourceLocation Loc, const char *&PrevSpec,
-                               unsigned &DiagID);
+                               unsigned &DiagID, ExplicitSpecifier ExplicitSpec,
+                               SourceLocation CloseParenLoc);
   bool setFunctionSpecNoreturn(SourceLocation Loc, const char *&PrevSpec,
                                unsigned &DiagID);
 

diff --git a/clang/include/clang/Sema/Overload.h b/clang/include/clang/Sema/Overload.h
@@ -705,6 +705,11 @@ class Sema;
     /// attribute disabled it.
     ovl_fail_enable_if,
 
+    /// This candidate constructor or conversion fonction
+    /// is used implicitly but the explicit(bool) specifier
+    /// was resolved to true
+    ovl_fail_explicit_resolved,
+
     /// This candidate was not viable because its address could not be taken.
     ovl_fail_addr_not_available,
 

diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
@@ -2743,7 +2743,8 @@ class Sema {
     CCEK_Enumerator,  ///< Enumerator value with fixed underlying type.
     CCEK_TemplateArg, ///< Value of a non-type template parameter.
     CCEK_NewExpr,     ///< Constant expression in a noptr-new-declarator.
-    CCEK_ConstexprIf  ///< Condition in a constexpr if statement.
+    CCEK_ConstexprIf, ///< Condition in a constexpr if statement.
+    CCEK_ExplicitBool ///< Condition in an explicit(bool) specifier.
   };
   ExprResult CheckConvertedConstantExpression(Expr *From, QualType T,
                                               llvm::APSInt &Value, CCEKind CCE);
@@ -2865,7 +2866,8 @@ class Sema {
                             OverloadCandidateSet &CandidateSet,
                             bool SuppressUserConversions = false,
                             bool PartialOverloading = false,
-                            bool AllowExplicit = false,
+                            bool AllowExplicit = true,
+                            bool AllowExplicitConversion = false,
                             ADLCallKind IsADLCandidate = ADLCallKind::NotADL,
                             ConversionSequenceList EarlyConversions = None);
   void AddFunctionCandidates(const UnresolvedSetImpl &Functions,
@@ -2904,7 +2906,7 @@ class Sema {
       FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
       TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
       OverloadCandidateSet &CandidateSet, bool SuppressUserConversions = false,
-      bool PartialOverloading = false,
+      bool PartialOverloading = false, bool AllowExplicit = true,
       ADLCallKind IsADLCandidate = ADLCallKind::NotADL);
   bool CheckNonDependentConversions(FunctionTemplateDecl *FunctionTemplate,
                                     ArrayRef<QualType> ParamTypes,
@@ -2916,20 +2918,16 @@ class Sema {
                                     QualType ObjectType = QualType(),
                                     Expr::Classification
                                         ObjectClassification = {});
-  void AddConversionCandidate(CXXConversionDecl *Conversion,
-                              DeclAccessPair FoundDecl,
-                              CXXRecordDecl *ActingContext,
-                              Expr *From, QualType ToType,
-                              OverloadCandidateSet& CandidateSet,
-                              bool AllowObjCConversionOnExplicit,
-                              bool AllowResultConversion = true);
-  void AddTemplateConversionCandidate(FunctionTemplateDecl *FunctionTemplate,
-                                      DeclAccessPair FoundDecl,
-                                      CXXRecordDecl *ActingContext,
-                                      Expr *From, QualType ToType,
-                                      OverloadCandidateSet &CandidateSet,
-                                      bool AllowObjCConversionOnExplicit,
-                                      bool AllowResultConversion = true);
+  void AddConversionCandidate(
+      CXXConversionDecl *Conversion, DeclAccessPair FoundDecl,
+      CXXRecordDecl *ActingContext, Expr *From, QualType ToType,
+      OverloadCandidateSet &CandidateSet, bool AllowObjCConversionOnExplicit,
+      bool AllowExplicit, bool AllowResultConversion = true);
+  void AddTemplateConversionCandidate(
+      FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
+      CXXRecordDecl *ActingContext, Expr *From, QualType ToType,
+      OverloadCandidateSet &CandidateSet, bool AllowObjCConversionOnExplicit,
+      bool AllowExplicit, bool AllowResultConversion = true);
   void AddSurrogateCandidate(CXXConversionDecl *Conversion,
                              DeclAccessPair FoundDecl,
                              CXXRecordDecl *ActingContext,
@@ -10158,6 +10156,14 @@ class Sema {
   ExprResult CheckBooleanCondition(SourceLocation Loc, Expr *E,
                                    bool IsConstexpr = false);
 
+  /// ActOnExplicitBoolSpecifier - Build an ExplicitSpecifier from an expression
+  /// found in an explicit(bool) specifier.
+  ExplicitSpecifier ActOnExplicitBoolSpecifier(Expr *E);
+
+  /// tryResolveExplicitSpecifier - Attempt to resolve the explict specifier.
+  /// Returns true if the explicit specifier is now resolved.
+  bool tryResolveExplicitSpecifier(ExplicitSpecifier &ExplicitSpec);
+
   /// DiagnoseAssignmentAsCondition - Given that an expression is
   /// being used as a boolean condition, warn if it's an assignment.
   void DiagnoseAssignmentAsCondition(Expr *E);

diff --git a/clang/include/clang/Serialization/ASTBitCodes.h b/clang/include/clang/Serialization/ASTBitCodes.h
@@ -1441,9 +1441,6 @@ namespace serialization {
       /// A CXXConstructorDecl record.
       DECL_CXX_CONSTRUCTOR,
 
-      /// A CXXConstructorDecl record for an inherited constructor.
-      DECL_CXX_INHERITED_CONSTRUCTOR,
-
       /// A CXXDestructorDecl record.
       DECL_CXX_DESTRUCTOR,
 

diff --git a/clang/include/clang/Serialization/ASTReader.h b/clang/include/clang/Serialization/ASTReader.h
@@ -2430,6 +2430,14 @@ class ASTRecordReader {
                                                  ID);
   }
 
+  ExplicitSpecifier readExplicitSpec() {
+    uint64_t Kind = readInt();
+    bool HasExpr = Kind & 0x1;
+    Kind = Kind >> 1;
+    return ExplicitSpecifier(HasExpr ? readExpr() : nullptr,
+                             static_cast<ExplicitSpecKind>(Kind));
+  }
+
   void readExceptionSpec(SmallVectorImpl<QualType> &ExceptionStorage,
                          FunctionProtoType::ExceptionSpecInfo &ESI) {
     return Reader->readExceptionSpec(*F, ExceptionStorage, ESI, Record, Idx);

diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
@@ -3076,11 +3076,20 @@ ExpectedDecl ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
   // Create the imported function.
   FunctionDecl *ToFunction = nullptr;
   if (auto *FromConstructor = dyn_cast<CXXConstructorDecl>(D)) {
+    Expr *ExplicitExpr = nullptr;
+    if (FromConstructor->getExplicitSpecifier().getExpr()) {
+      auto Imp = importSeq(FromConstructor->getExplicitSpecifier().getExpr());
+      if (!Imp)
+        return Imp.takeError();
+      std::tie(ExplicitExpr) = *Imp;
+    }
     if (GetImportedOrCreateDecl<CXXConstructorDecl>(
-        ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
-        ToInnerLocStart, NameInfo, T, TInfo,
-        FromConstructor->isExplicit(),
-        D->isInlineSpecified(), D->isImplicit(), D->isConstexpr()))
+            ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
+            ToInnerLocStart, NameInfo, T, TInfo,
+            ExplicitSpecifier(
+                ExplicitExpr,
+                FromConstructor->getExplicitSpecifier().getKind()),
+            D->isInlineSpecified(), D->isImplicit(), D->isConstexpr()))
       return ToFunction;
   } else if (CXXDestructorDecl *FromDtor = dyn_cast<CXXDestructorDecl>(D)) {
 
@@ -3106,10 +3115,19 @@ ExpectedDecl ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
     ToDtor->setOperatorDelete(ToOperatorDelete, ToThisArg);
   } else if (CXXConversionDecl *FromConversion =
                  dyn_cast<CXXConversionDecl>(D)) {
+    Expr *ExplicitExpr = nullptr;
+    if (FromConversion->getExplicitSpecifier().getExpr()) {
+      auto Imp = importSeq(FromConversion->getExplicitSpecifier().getExpr());
+      if (!Imp)
+        return Imp.takeError();
+      std::tie(ExplicitExpr) = *Imp;
+    }
     if (GetImportedOrCreateDecl<CXXConversionDecl>(
             ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
             ToInnerLocStart, NameInfo, T, TInfo, D->isInlineSpecified(),
-            FromConversion->isExplicit(), D->isConstexpr(), SourceLocation()))
+            ExplicitSpecifier(ExplicitExpr,
+                              FromConversion->getExplicitSpecifier().getKind()),
+            D->isConstexpr(), SourceLocation()))
       return ToFunction;
   } else if (auto *Method = dyn_cast<CXXMethodDecl>(D)) {
     if (GetImportedOrCreateDecl<CXXMethodDecl>(

diff --git a/clang/lib/AST/ASTStructuralEquivalence.cpp b/clang/lib/AST/ASTStructuralEquivalence.cpp
@@ -982,13 +982,15 @@ static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
 
   if (auto *Constructor1 = dyn_cast<CXXConstructorDecl>(Method1)) {
     auto *Constructor2 = cast<CXXConstructorDecl>(Method2);
-    if (Constructor1->isExplicit() != Constructor2->isExplicit())
+    if (!Constructor1->getExplicitSpecifier().isEquivalent(
+            Constructor2->getExplicitSpecifier()))
       return false;
   }
 
   if (auto *Conversion1 = dyn_cast<CXXConversionDecl>(Method1)) {
     auto *Conversion2 = cast<CXXConversionDecl>(Method2);
-    if (Conversion1->isExplicit() != Conversion2->isExplicit())
+    if (!Conversion1->getExplicitSpecifier().isEquivalent(
+            Conversion2->getExplicitSpecifier()))
       return false;
     if (!IsStructurallyEquivalent(Context, Conversion1->getConversionType(),
                                   Conversion2->getConversionType()))

diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
@@ -2713,7 +2713,6 @@ FunctionDecl::FunctionDecl(Kind DK, ASTContext &C, DeclContext *DC,
   FunctionDeclBits.SClass = S;
   FunctionDeclBits.IsInline = isInlineSpecified;
   FunctionDeclBits.IsInlineSpecified = isInlineSpecified;
-  FunctionDeclBits.IsExplicitSpecified = false;
   FunctionDeclBits.IsVirtualAsWritten = false;
   FunctionDeclBits.IsPure = false;
   FunctionDeclBits.HasInheritedPrototype = false;

diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
@@ -1879,19 +1879,47 @@ bool CXXRecordDecl::mayBeAbstract() const {
 
 void CXXDeductionGuideDecl::anchor() {}
 
+bool ExplicitSpecifier::isEquivalent(const ExplicitSpecifier Other) const {
+  if ((getKind() != Other.getKind() ||
+       getKind() == ExplicitSpecKind::Unresolved)) {
+    if (getKind() == ExplicitSpecKind::Unresolved &&
+        Other.getKind() == ExplicitSpecKind::Unresolved) {
+      ODRHash SelfHash, OtherHash;
+      SelfHash.AddStmt(getExpr());
+      OtherHash.AddStmt(Other.getExpr());
+      return SelfHash.CalculateHash() == OtherHash.CalculateHash();
+    } else
+      return false;
+  }
+  return true;
+}
+
+ExplicitSpecifier ExplicitSpecifier::getFromDecl(FunctionDecl *Function) {
+  switch (Function->getDeclKind()) {
+  case Decl::Kind::CXXConstructor:
+    return cast<CXXConstructorDecl>(Function)->getExplicitSpecifier();
+  case Decl::Kind::CXXConversion:
+    return cast<CXXConversionDecl>(Function)->getExplicitSpecifier();
+  case Decl::Kind::CXXDeductionGuide:
+    return cast<CXXDeductionGuideDecl>(Function)->getExplicitSpecifier();
+  default:
+    return {};
+  }
+}
+
 CXXDeductionGuideDecl *CXXDeductionGuideDecl::Create(
-    ASTContext &C, DeclContext *DC, SourceLocation StartLoc, bool IsExplicit,
-    const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
-    SourceLocation EndLocation) {
-  return new (C, DC) CXXDeductionGuideDecl(C, DC, StartLoc, IsExplicit,
-                                           NameInfo, T, TInfo, EndLocation);
+    ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
+    ExplicitSpecifier ES, const DeclarationNameInfo &NameInfo, QualType T,
+    TypeSourceInfo *TInfo, SourceLocation EndLocation) {
+  return new (C, DC) CXXDeductionGuideDecl(C, DC, StartLoc, ES, NameInfo, T,
+                                           TInfo, EndLocation);
 }
 
 CXXDeductionGuideDecl *CXXDeductionGuideDecl::CreateDeserialized(ASTContext &C,
                                                                  unsigned ID) {
-  return new (C, ID) CXXDeductionGuideDecl(C, nullptr, SourceLocation(), false,
-                                           DeclarationNameInfo(), QualType(),
-                                           nullptr, SourceLocation());
+  return new (C, ID) CXXDeductionGuideDecl(
+      C, nullptr, SourceLocation(), ExplicitSpecifier(), DeclarationNameInfo(),
+      QualType(), nullptr, SourceLocation());
 }
 
 void CXXMethodDecl::anchor() {}
@@ -2329,47 +2357,54 @@ SourceRange CXXCtorInitializer::getSourceRange() const {
 CXXConstructorDecl::CXXConstructorDecl(
     ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
     const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
-    bool isExplicitSpecified, bool isInline, bool isImplicitlyDeclared,
+    ExplicitSpecifier ES, bool isInline, bool isImplicitlyDeclared,
     bool isConstexpr, InheritedConstructor Inherited)
     : CXXMethodDecl(CXXConstructor, C, RD, StartLoc, NameInfo, T, TInfo,
                     SC_None, isInline, isConstexpr, SourceLocation()) {
   setNumCtorInitializers(0);
   setInheritingConstructor(static_cast<bool>(Inherited));
   setImplicit(isImplicitlyDeclared);
+  CXXConstructorDeclBits.HasTrailingExplicitSpecifier = ES.getExpr() ? 1 : 0;
   if (Inherited)
     *getTrailingObjects<InheritedConstructor>() = Inherited;
-  setExplicitSpecified(isExplicitSpecified);
+  setExplicitSpecifier(ES);
 }
 
 void CXXConstructorDecl::anchor() {}
 
 CXXConstructorDecl *CXXConstructorDecl::CreateDeserialized(ASTContext &C,
                                                            unsigned ID,
-                                                           bool Inherited) {
-  unsigned Extra = additionalSizeToAlloc<InheritedConstructor>(Inherited);
+                                                           uint64_t AllocKind) {
+  bool hasTraillingExplicit = static_cast<bool>(AllocKind & TAKHasTailExplicit);
+  bool isInheritingConstructor =
+      static_cast<bool>(AllocKind & TAKInheritsConstructor);
+  unsigned Extra =
+      additionalSizeToAlloc<InheritedConstructor, ExplicitSpecifier>(
+          isInheritingConstructor, hasTraillingExplicit);
   auto *Result = new (C, ID, Extra) CXXConstructorDecl(
       C, nullptr, SourceLocation(), DeclarationNameInfo(), QualType(), nullptr,
-      false, false, false, false, InheritedConstructor());
-  Result->setInheritingConstructor(Inherited);
+      ExplicitSpecifier(), false, false, false, InheritedConstructor());
+  Result->setInheritingConstructor(isInheritingConstructor);
+  Result->CXXConstructorDeclBits.HasTrailingExplicitSpecifier =
+      hasTraillingExplicit;
+  Result->setExplicitSpecifier(ExplicitSpecifier());
   return Result;
 }
 
-CXXConstructorDecl *
-CXXConstructorDecl::Create(ASTContext &C, CXXRecordDecl *RD,
-                           SourceLocation StartLoc,
-                           const DeclarationNameInfo &NameInfo,
-                           QualType T, TypeSourceInfo *TInfo,
-                           bool isExplicit, bool isInline,
-                           bool isImplicitlyDeclared, bool isConstexpr,
-                           InheritedConstructor Inherited) {
+CXXConstructorDecl *CXXConstructorDecl::Create(
+    ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
+    const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
+    ExplicitSpecifier ES, bool isInline, bool isImplicitlyDeclared,
+    bool isConstexpr, InheritedConstructor Inherited) {
   assert(NameInfo.getName().getNameKind()
          == DeclarationName::CXXConstructorName &&
          "Name must refer to a constructor");
   unsigned Extra =
-      additionalSizeToAlloc<InheritedConstructor>(Inherited ? 1 : 0);
-  return new (C, RD, Extra) CXXConstructorDecl(
-      C, RD, StartLoc, NameInfo, T, TInfo, isExplicit, isInline,
-      isImplicitlyDeclared, isConstexpr, Inherited);
+      additionalSizeToAlloc<InheritedConstructor, ExplicitSpecifier>(
+          Inherited ? 1 : 0, ES.getExpr() ? 1 : 0);
+  return new (C, RD, Extra)
+      CXXConstructorDecl(C, RD, StartLoc, NameInfo, T, TInfo, ES, isInline,
+                         isImplicitlyDeclared, isConstexpr, Inherited);
 }
 
 CXXConstructorDecl::init_const_iterator CXXConstructorDecl::init_begin() const {
@@ -2520,25 +2555,21 @@ void CXXConversionDecl::anchor() {}
 
 CXXConversionDecl *
 CXXConversionDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) CXXConversionDecl(C, nullptr, SourceLocation(),
-                                       DeclarationNameInfo(), QualType(),
-                                       nullptr, false, false, false,
-                                       SourceLocation());
+  return new (C, ID) CXXConversionDecl(
+      C, nullptr, SourceLocation(), DeclarationNameInfo(), QualType(), nullptr,
+      false, ExplicitSpecifier(), false, SourceLocation());
 }
 
-CXXConversionDecl *
-CXXConversionDecl::Create(ASTContext &C, CXXRecordDecl *RD,
-                          SourceLocation StartLoc,
-                          const DeclarationNameInfo &NameInfo,
-                          QualType T, TypeSourceInfo *TInfo,
-                          bool isInline, bool isExplicit,
-                          bool isConstexpr, SourceLocation EndLocation) {
+CXXConversionDecl *CXXConversionDecl::Create(
+    ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
+    const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
+    bool isInline, ExplicitSpecifier ES, bool isConstexpr,
+    SourceLocation EndLocation) {
   assert(NameInfo.getName().getNameKind()
          == DeclarationName::CXXConversionFunctionName &&
          "Name must refer to a conversion function");
   return new (C, RD) CXXConversionDecl(C, RD, StartLoc, NameInfo, T, TInfo,
-                                       isInline, isExplicit, isConstexpr,
-                                       EndLocation);
+                                       isInline, ES, isConstexpr, EndLocation);
 }
 
 bool CXXConversionDecl::isLambdaToBlockPointerConversion() const {

diff --git a/clang/lib/AST/DeclPrinter.cpp b/clang/lib/AST/DeclPrinter.cpp
@@ -566,6 +566,21 @@ void DeclPrinter::VisitEnumConstantDecl(EnumConstantDecl *D) {
   }
 }
 
+static void printExplicitSpecifier(ExplicitSpecifier ES, llvm::raw_ostream &Out,
+                                   PrintingPolicy &Policy,
+                                   unsigned Indentation) {
+  std::string Proto = "explicit";
+  llvm::raw_string_ostream EOut(Proto);
+  if (ES.getExpr()) {
+    EOut << "(";
+    ES.getExpr()->printPretty(EOut, nullptr, Policy, Indentation);
+    EOut << ")";
+  }
+  EOut << " ";
+  EOut.flush();
+  Out << EOut.str();
+}
+
 void DeclPrinter::VisitFunctionDecl(FunctionDecl *D) {
   if (!D->getDescribedFunctionTemplate() &&
       !D->isFunctionTemplateSpecialization())
@@ -596,10 +611,9 @@ void DeclPrinter::VisitFunctionDecl(FunctionDecl *D) {
     if (D->isVirtualAsWritten()) Out << "virtual ";
     if (D->isModulePrivate())    Out << "__module_private__ ";
     if (D->isConstexpr() && !D->isExplicitlyDefaulted()) Out << "constexpr ";
-    if ((CDecl && CDecl->isExplicitSpecified()) ||
-        (ConversionDecl && ConversionDecl->isExplicitSpecified()) ||
-        (GuideDecl && GuideDecl->isExplicitSpecified()))
-      Out << "explicit ";
+    ExplicitSpecifier ExplicitSpec = ExplicitSpecifier::getFromDecl(D);
+    if (ExplicitSpec.isSpecified())
+      printExplicitSpecifier(ExplicitSpec, Out, Policy, Indentation);
   }
 
   PrintingPolicy SubPolicy(Policy);

diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -540,6 +540,8 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_template_template_args", "201611L");
 
   // C++20 features.
+  if (LangOpts.CPlusPlus2a)
+    Builder.defineMacro("__cpp_conditional_explicit", "201806L");
   if (LangOpts.Char8)
     Builder.defineMacro("__cpp_char8_t", "201811L");
   Builder.defineMacro("__cpp_impl_destroying_delete", "201806L");

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
@@ -2480,12 +2480,12 @@ void Parser::ParseSpecifierQualifierList(DeclSpec &DS, AccessSpecifier AS,
       Diag(DS.getInlineSpecLoc(), diag::err_typename_invalid_functionspec);
     if (DS.isVirtualSpecified())
       Diag(DS.getVirtualSpecLoc(), diag::err_typename_invalid_functionspec);
-    if (DS.isExplicitSpecified())
+    if (DS.hasExplicitSpecifier())
       Diag(DS.getExplicitSpecLoc(), diag::err_typename_invalid_functionspec);
     DS.ClearFunctionSpecs();
   }
 
-  // Issue diagnostic and remove constexpr specfier if present.
+  // Issue diagnostic and remove constexpr specifier if present.
   if (DS.isConstexprSpecified() && DSC != DeclSpecContext::DSC_condition) {
     Diag(DS.getConstexprSpecLoc(), diag::err_typename_invalid_constexpr);
     DS.ClearConstexprSpec();
@@ -3004,9 +3004,13 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
   while (1) {
     bool isInvalid = false;
     bool isStorageClass = false;
+    bool isAlreadyConsumed = false;
     const char *PrevSpec = nullptr;
     unsigned DiagID = 0;
 
+    // This value need to be set when isAlreadyConsumed is set to true.
+    SourceLocation RangeEnd;
+
     // HACK: MSVC doesn't consider _Atomic to be a keyword and its STL
     // implementation for VS2013 uses _Atomic as an identifier for one of the
     // classes in <atomic>.
@@ -3558,9 +3562,34 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
         isInvalid = DS.setFunctionSpecVirtual(Loc, PrevSpec, DiagID);
       }
       break;
-    case tok::kw_explicit:
-      isInvalid = DS.setFunctionSpecExplicit(Loc, PrevSpec, DiagID);
+    case tok::kw_explicit: {
+      SourceLocation ExplicitLoc = Loc;
+      SourceLocation CloseParenLoc;
+      ExplicitSpecifier ExplicitSpec(nullptr, ExplicitSpecKind::ResolvedTrue);
+      isAlreadyConsumed = true;
+      RangeEnd = ExplicitLoc;
+      ConsumeToken(); // kw_explicit
+      if (Tok.is(tok::l_paren)) {
+        if (getLangOpts().CPlusPlus2a) {
+          ExprResult ExplicitExpr(static_cast<Expr *>(nullptr));
+          BalancedDelimiterTracker Tracker(*this, tok::l_paren);
+          Tracker.consumeOpen();
+          ExplicitExpr = ParseConstantExpression();
+          RangeEnd = Tok.getLocation();
+          if (ExplicitExpr.isUsable()) {
+            CloseParenLoc = Tok.getLocation();
+            Tracker.consumeClose();
+            ExplicitSpec =
+                Actions.ActOnExplicitBoolSpecifier(ExplicitExpr.get());
+          } else
+            Tracker.skipToEnd();
+        } else
+          Diag(Tok.getLocation(), diag::warn_cxx2a_compat_explicit_bool);
+      }
+      isInvalid = DS.setFunctionSpecExplicit(ExplicitLoc, PrevSpec, DiagID,
+                                             ExplicitSpec, CloseParenLoc);
       break;
+    }
     case tok::kw__Noreturn:
       if (!getLangOpts().C11)
         Diag(Loc, diag::ext_c11_noreturn);
@@ -3904,25 +3933,32 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       // If a type specifier follows, it will be diagnosed elsewhere.
       continue;
     }
+
+    assert(!isAlreadyConsumed || RangeEnd != SourceLocation() &&
+                                     "both or neither of isAlreadyConsumed and "
+                                     "RangeEnd needs to be set");
+    DS.SetRangeEnd(isAlreadyConsumed ? RangeEnd : Tok.getLocation());
+
     // If the specifier wasn't legal, issue a diagnostic.
     if (isInvalid) {
       assert(PrevSpec && "Method did not return previous specifier!");
       assert(DiagID);
 
       if (DiagID == diag::ext_duplicate_declspec ||
-          DiagID == diag::ext_warn_duplicate_declspec)
-        Diag(Tok, DiagID)
-          << PrevSpec << FixItHint::CreateRemoval(Tok.getLocation());
+          DiagID == diag::ext_warn_duplicate_declspec ||
+          DiagID == diag::err_duplicate_declspec)
+        Diag(Loc, DiagID) << PrevSpec
+                          << FixItHint::CreateRemoval(
+                                 SourceRange(Loc, DS.getEndLoc()));
       else if (DiagID == diag::err_opencl_unknown_type_specifier) {
-        Diag(Tok, DiagID) << getLangOpts().OpenCLCPlusPlus
-            << getLangOpts().getOpenCLVersionTuple().getAsString()
-            << PrevSpec << isStorageClass;
+        Diag(Loc, DiagID) << getLangOpts().OpenCLCPlusPlus
+                          << getLangOpts().getOpenCLVersionTuple().getAsString()
+                          << PrevSpec << isStorageClass;
       } else
-        Diag(Tok, DiagID) << PrevSpec;
+        Diag(Loc, DiagID) << PrevSpec;
     }
 
-    DS.SetRangeEnd(Tok.getLocation());
-    if (DiagID != diag::err_bool_redeclaration)
+    if (DiagID != diag::err_bool_redeclaration && !isAlreadyConsumed)
       // After an error the next token can be an annotation token.
       ConsumeAnyToken();
 

diff --git a/clang/lib/Sema/DeclSpec.cpp b/clang/lib/Sema/DeclSpec.cpp
@@ -445,7 +445,7 @@ unsigned DeclSpec::getParsedSpecifiers() const {
   if (hasTypeSpecifier())
     Res |= PQ_TypeSpecifier;
 
-  if (FS_inline_specified || FS_virtual_specified || FS_explicit_specified ||
+  if (FS_inline_specified || FS_virtual_specified || hasExplicitSpecifier() ||
       FS_noreturn_specified || FS_forceinline_specified)
     Res |= PQ_FunctionSpecifier;
   return Res;
@@ -959,17 +959,24 @@ bool DeclSpec::setFunctionSpecVirtual(SourceLocation Loc,
 }
 
 bool DeclSpec::setFunctionSpecExplicit(SourceLocation Loc,
-                                       const char *&PrevSpec,
-                                       unsigned &DiagID) {
+                                       const char *&PrevSpec, unsigned &DiagID,
+                                       ExplicitSpecifier ExplicitSpec,
+                                       SourceLocation CloseParenLoc) {
+  assert((ExplicitSpec.getKind() == ExplicitSpecKind::ResolvedTrue ||
+          ExplicitSpec.getExpr()) &&
+         "invalid ExplicitSpecifier");
   // 'explicit explicit' is ok, but warn as this is likely not what the user
   // intended.
-  if (FS_explicit_specified) {
-    DiagID = diag::warn_duplicate_declspec;
+  if (hasExplicitSpecifier()) {
+    DiagID = (ExplicitSpec.getExpr() || FS_explicit_specifier.getExpr())
+                 ? diag::err_duplicate_declspec
+                 : diag::ext_warn_duplicate_declspec;
     PrevSpec = "explicit";
     return true;
   }
-  FS_explicit_specified = true;
+  FS_explicit_specifier = ExplicitSpec;
   FS_explicitLoc = Loc;
+  FS_explicitCloseParenLoc = CloseParenLoc;
   return false;
 }
 
@@ -1311,23 +1318,26 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
   //   The explicit specifier shall be used only in the declaration of
   //   a constructor or conversion function within its class
   //   definition;
-  if (isFriendSpecified() && (isVirtualSpecified() || isExplicitSpecified())) {
+  if (isFriendSpecified() && (isVirtualSpecified() || hasExplicitSpecifier())) {
     StringRef Keyword;
+    FixItHint Hint;
     SourceLocation SCLoc;
 
     if (isVirtualSpecified()) {
       Keyword = "virtual";
       SCLoc = getVirtualSpecLoc();
+      Hint = FixItHint::CreateRemoval(SCLoc);
     } else {
       Keyword = "explicit";
       SCLoc = getExplicitSpecLoc();
+      Hint = FixItHint::CreateRemoval(getExplicitSpecRange());
     }
 
-    FixItHint Hint = FixItHint::CreateRemoval(SCLoc);
     S.Diag(SCLoc, diag::err_friend_decl_spec)
       << Keyword << Hint;
 
-    FS_virtual_specified = FS_explicit_specified = false;
+    FS_virtual_specified = false;
+    FS_explicit_specifier = ExplicitSpecifier();
     FS_virtualLoc = FS_explicitLoc = SourceLocation();
   }
 

diff --git a/clang/lib/Sema/SemaCodeComplete.cpp b/clang/lib/Sema/SemaCodeComplete.cpp
@@ -4970,7 +4970,8 @@ QualType Sema::ProduceConstructorSignatureHelp(Scope *S, QualType Type,
       AddOverloadCandidate(FD, DeclAccessPair::make(FD, C->getAccess()), Args,
                            CandidateSet,
                            /*SuppressUsedConversions=*/false,
-                           /*PartialOverloading=*/true);
+                           /*PartialOverloading=*/true,
+                           /*AllowExplicit*/ true);
     } else if (auto *FTD = dyn_cast<FunctionTemplateDecl>(C)) {
       AddTemplateOverloadCandidate(
           FTD, DeclAccessPair::make(FTD, C->getAccess()),

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
@@ -5725,7 +5725,7 @@ void Sema::DiagnoseFunctionSpecifiers(const DeclSpec &DS) {
     Diag(DS.getVirtualSpecLoc(),
          diag::err_virtual_non_function);
 
-  if (DS.isExplicitSpecified())
+  if (DS.hasExplicitSpecifier())
     Diag(DS.getExplicitSpecLoc(),
          diag::err_explicit_non_function);
 
@@ -7989,7 +7989,7 @@ static FunctionDecl* CreateNewFunctionDecl(Sema &SemaRef, Declarator &D,
     return NewFD;
   }
 
-  bool isExplicit = D.getDeclSpec().isExplicitSpecified();
+  ExplicitSpecifier ExplicitSpecifier = D.getDeclSpec().getExplicitSpecifier();
   bool isConstexpr = D.getDeclSpec().isConstexprSpecified();
 
   // Check that the return type is not an abstract class type.
@@ -8009,7 +8009,7 @@ static FunctionDecl* CreateNewFunctionDecl(Sema &SemaRef, Declarator &D,
     R = SemaRef.CheckConstructorDeclarator(D, R, SC);
     return CXXConstructorDecl::Create(
         SemaRef.Context, cast<CXXRecordDecl>(DC), D.getBeginLoc(), NameInfo, R,
-        TInfo, isExplicit, isInline,
+        TInfo, ExplicitSpecifier, isInline,
         /*isImplicitlyDeclared=*/false, isConstexpr);
 
   } else if (Name.getNameKind() == DeclarationName::CXXDestructorName) {
@@ -8054,13 +8054,13 @@ static FunctionDecl* CreateNewFunctionDecl(Sema &SemaRef, Declarator &D,
     IsVirtualOkay = true;
     return CXXConversionDecl::Create(
         SemaRef.Context, cast<CXXRecordDecl>(DC), D.getBeginLoc(), NameInfo, R,
-        TInfo, isInline, isExplicit, isConstexpr, SourceLocation());
+        TInfo, isInline, ExplicitSpecifier, isConstexpr, SourceLocation());
 
   } else if (Name.getNameKind() == DeclarationName::CXXDeductionGuideName) {
     SemaRef.CheckDeductionGuideDeclarator(D, R, SC);
 
     return CXXDeductionGuideDecl::Create(SemaRef.Context, DC, D.getBeginLoc(),
-                                         isExplicit, NameInfo, R, TInfo,
+                                         ExplicitSpecifier, NameInfo, R, TInfo,
                                          D.getEndLoc());
   } else if (DC->isRecord()) {
     // If the name of the function is the same as the name of the record,
@@ -8421,7 +8421,7 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
   if (getLangOpts().CPlusPlus) {
     bool isInline = D.getDeclSpec().isInlineSpecified();
     bool isVirtual = D.getDeclSpec().isVirtualSpecified();
-    bool isExplicit = D.getDeclSpec().isExplicitSpecified();
+    bool hasExplicit = D.getDeclSpec().hasExplicitSpecifier();
     bool isConstexpr = D.getDeclSpec().isConstexprSpecified();
     isFriend = D.getDeclSpec().isFriendSpecified();
     if (isFriend && !isInline && D.isFunctionDefinition()) {
@@ -8604,20 +8604,20 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
     //  The explicit specifier shall be used only in the declaration of a
     //  constructor or conversion function within its class definition;
     //  see 12.3.1 and 12.3.2.
-    if (isExplicit && !NewFD->isInvalidDecl() &&
+    if (hasExplicit && !NewFD->isInvalidDecl() &&
         !isa<CXXDeductionGuideDecl>(NewFD)) {
       if (!CurContext->isRecord()) {
         // 'explicit' was specified outside of the class.
         Diag(D.getDeclSpec().getExplicitSpecLoc(),
              diag::err_explicit_out_of_class)
-          << FixItHint::CreateRemoval(D.getDeclSpec().getExplicitSpecLoc());
+            << FixItHint::CreateRemoval(D.getDeclSpec().getExplicitSpecRange());
       } else if (!isa<CXXConstructorDecl>(NewFD) &&
                  !isa<CXXConversionDecl>(NewFD)) {
         // 'explicit' was specified on a function that wasn't a constructor
         // or conversion function.
         Diag(D.getDeclSpec().getExplicitSpecLoc(),
              diag::err_explicit_non_ctor_or_conv_function)
-          << FixItHint::CreateRemoval(D.getDeclSpec().getExplicitSpecLoc());
+            << FixItHint::CreateRemoval(D.getDeclSpec().getExplicitSpecRange());
       }
     }
 

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -657,14 +657,13 @@ bool Sema::MergeCXXFunctionDecl(FunctionDecl *New, FunctionDecl *Old,
     Invalid = true;
   }
 
-  // FIXME: It's not clear what should happen if multiple declarations of a
-  // deduction guide have different explicitness. For now at least we simply
-  // reject any case where the explicitness changes.
-  auto *NewGuide = dyn_cast<CXXDeductionGuideDecl>(New);
-  if (NewGuide && NewGuide->isExplicitSpecified() !=
-                      cast<CXXDeductionGuideDecl>(Old)->isExplicitSpecified()) {
-    Diag(New->getLocation(), diag::err_deduction_guide_explicit_mismatch)
-      << NewGuide->isExplicitSpecified();
+  // C++17 [temp.deduct.guide]p3:
+  //   Two deduction guide declarations in the same translation unit
+  //   for the same class template shall not have equivalent
+  //   parameter-declaration-clauses.
+  if (isa<CXXDeductionGuideDecl>(New) &&
+      !New->isFunctionTemplateSpecialization()) {
+    Diag(New->getLocation(), diag::err_deduction_guide_redeclared);
     Diag(Old->getLocation(), diag::note_previous_declaration);
   }
 
@@ -8594,12 +8593,12 @@ void Sema::CheckConversionDeclarator(Declarator &D, QualType &R,
     R = Context.getFunctionType(ConvType, None, Proto->getExtProtoInfo());
 
   // C++0x explicit conversion operators.
-  if (DS.isExplicitSpecified())
+  if (DS.hasExplicitSpecifier() && !getLangOpts().CPlusPlus2a)
     Diag(DS.getExplicitSpecLoc(),
          getLangOpts().CPlusPlus11
              ? diag::warn_cxx98_compat_explicit_conversion_functions
              : diag::ext_explicit_conversion_functions)
-        << SourceRange(DS.getExplicitSpecLoc());
+        << SourceRange(DS.getExplicitSpecRange());
 }
 
 /// ActOnConversionDeclarator - Called by ActOnDeclarator to complete
@@ -10822,6 +10821,28 @@ struct ComputingExceptionSpec {
 };
 }
 
+bool Sema::tryResolveExplicitSpecifier(ExplicitSpecifier &ExplicitSpec) {
+  llvm::APSInt Result;
+  ExprResult Converted = CheckConvertedConstantExpression(
+      ExplicitSpec.getExpr(), Context.BoolTy, Result, CCEK_ExplicitBool);
+  ExplicitSpec.setExpr(Converted.get());
+  if (Converted.isUsable() && !Converted.get()->isValueDependent()) {
+    ExplicitSpec.setKind(Result.getBoolValue()
+                             ? ExplicitSpecKind::ResolvedTrue
+                             : ExplicitSpecKind::ResolvedFalse);
+    return true;
+  }
+  ExplicitSpec.setKind(ExplicitSpecKind::Unresolved);
+  return false;
+}
+
+ExplicitSpecifier Sema::ActOnExplicitBoolSpecifier(Expr *ExplicitExpr) {
+  ExplicitSpecifier ES(ExplicitExpr, ExplicitSpecKind::Unresolved);
+  if (!ExplicitExpr->isTypeDependent())
+    tryResolveExplicitSpecifier(ES);
+  return ES;
+}
+
 static Sema::ImplicitExceptionSpecification
 ComputeDefaultedSpecialMemberExceptionSpec(
     Sema &S, SourceLocation Loc, CXXMethodDecl *MD, Sema::CXXSpecialMember CSM,
@@ -10970,9 +10991,9 @@ CXXConstructorDecl *Sema::DeclareImplicitDefaultConstructor(
     = Context.DeclarationNames.getCXXConstructorName(ClassType);
   DeclarationNameInfo NameInfo(Name, ClassLoc);
   CXXConstructorDecl *DefaultCon = CXXConstructorDecl::Create(
-      Context, ClassDecl, ClassLoc, NameInfo, /*Type*/QualType(),
-      /*TInfo=*/nullptr, /*isExplicit=*/false, /*isInline=*/true,
-      /*isImplicitlyDeclared=*/true, Constexpr);
+      Context, ClassDecl, ClassLoc, NameInfo, /*Type*/ QualType(),
+      /*TInfo=*/nullptr, ExplicitSpecifier(),
+      /*isInline=*/true, /*isImplicitlyDeclared=*/true, Constexpr);
   DefaultCon->setAccess(AS_public);
   DefaultCon->setDefaulted();
 
@@ -11091,7 +11112,7 @@ Sema::findInheritingConstructor(SourceLocation Loc,
 
   CXXConstructorDecl *DerivedCtor = CXXConstructorDecl::Create(
       Context, Derived, UsingLoc, NameInfo, TInfo->getType(), TInfo,
-      BaseCtor->isExplicit(), /*Inline=*/true,
+      BaseCtor->getExplicitSpecifier(), /*Inline=*/true,
       /*ImplicitlyDeclared=*/true, Constexpr,
       InheritedConstructor(Shadow, BaseCtor));
   if (Shadow->isInvalidDecl())
@@ -12543,8 +12564,9 @@ CXXConstructorDecl *Sema::DeclareImplicitCopyConstructor(
   //   member of its class.
   CXXConstructorDecl *CopyConstructor = CXXConstructorDecl::Create(
       Context, ClassDecl, ClassLoc, NameInfo, QualType(), /*TInfo=*/nullptr,
-      /*isExplicit=*/false, /*isInline=*/true, /*isImplicitlyDeclared=*/true,
-      Constexpr);
+      ExplicitSpecifier(),
+      /*isInline=*/true,
+      /*isImplicitlyDeclared=*/true, Constexpr);
   CopyConstructor->setAccess(AS_public);
   CopyConstructor->setDefaulted();
 
@@ -12673,8 +12695,9 @@ CXXConstructorDecl *Sema::DeclareImplicitMoveConstructor(
   //   member of its class.
   CXXConstructorDecl *MoveConstructor = CXXConstructorDecl::Create(
       Context, ClassDecl, ClassLoc, NameInfo, QualType(), /*TInfo=*/nullptr,
-      /*isExplicit=*/false, /*isInline=*/true, /*isImplicitlyDeclared=*/true,
-      Constexpr);
+      ExplicitSpecifier(),
+      /*isInline=*/true,
+      /*isImplicitlyDeclared=*/true, Constexpr);
   MoveConstructor->setAccess(AS_public);
   MoveConstructor->setDefaulted();
 

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
@@ -3763,9 +3763,10 @@ ResolveConstructorOverload(Sema &S, SourceLocation DeclLoc,
          hasCopyOrMoveCtorParam(S.Context, Info));
 
     if (Info.ConstructorTmpl)
-      S.AddTemplateOverloadCandidate(Info.ConstructorTmpl, Info.FoundDecl,
-                                     /*ExplicitArgs*/ nullptr, Args,
-                                     CandidateSet, SuppressUserConversions);
+      S.AddTemplateOverloadCandidate(
+          Info.ConstructorTmpl, Info.FoundDecl,
+          /*ExplicitArgs*/ nullptr, Args, CandidateSet, SuppressUserConversions,
+          /*PartialOverloading=*/false, AllowExplicit);
     else {
       // C++ [over.match.copy]p1:
       //   - When initializing a temporary to be bound to the first parameter
@@ -3779,8 +3780,8 @@ ResolveConstructorOverload(Sema &S, SourceLocation DeclLoc,
                                hasCopyOrMoveCtorParam(S.Context, Info);
       S.AddOverloadCandidate(Info.Constructor, Info.FoundDecl, Args,
                              CandidateSet, SuppressUserConversions,
-                             /*PartialOverloading=*/false,
-                             /*AllowExplicit=*/AllowExplicitConv);
+                             /*PartialOverloading=*/false, AllowExplicit,
+                             AllowExplicitConv);
     }
   }
 
@@ -3813,16 +3814,17 @@ ResolveConstructorOverload(Sema &S, SourceLocation DeclLoc,
         else
           Conv = cast<CXXConversionDecl>(D);
 
-        if ((AllowExplicit && !CopyInitializing) || !Conv->isExplicit()) {
+        if (AllowExplicit || !Conv->isExplicit()) {
           if (ConvTemplate)
-            S.AddTemplateConversionCandidate(ConvTemplate, I.getPair(),
-                                             ActingDC, Initializer, DestType,
-                                             CandidateSet, AllowExplicit,
-                                             /*AllowResultConversion*/false);
+            S.AddTemplateConversionCandidate(
+                ConvTemplate, I.getPair(), ActingDC, Initializer, DestType,
+                CandidateSet, AllowExplicit, AllowExplicit,
+                /*AllowResultConversion*/ false);
           else
             S.AddConversionCandidate(Conv, I.getPair(), ActingDC, Initializer,
                                      DestType, CandidateSet, AllowExplicit,
-                                     /*AllowResultConversion*/false);
+                                     AllowExplicit,
+                                     /*AllowResultConversion*/ false);
         }
       }
     }
@@ -4368,14 +4370,16 @@ static OverloadingResult TryRefInitWithConversionFunction(
       if (!Info.Constructor->isInvalidDecl() &&
           Info.Constructor->isConvertingConstructor(AllowExplicitCtors)) {
         if (Info.ConstructorTmpl)
-          S.AddTemplateOverloadCandidate(Info.ConstructorTmpl, Info.FoundDecl,
-                                         /*ExplicitArgs*/ nullptr,
-                                         Initializer, CandidateSet,
-                                         /*SuppressUserConversions=*/true);
+          S.AddTemplateOverloadCandidate(
+              Info.ConstructorTmpl, Info.FoundDecl,
+              /*ExplicitArgs*/ nullptr, Initializer, CandidateSet,
+              /*SuppressUserConversions=*/true,
+              /*PartialOverloading*/ false, AllowExplicitCtors);
         else
-          S.AddOverloadCandidate(Info.Constructor, Info.FoundDecl,
-                                 Initializer, CandidateSet,
-                                 /*SuppressUserConversions=*/true);
+          S.AddOverloadCandidate(
+              Info.Constructor, Info.FoundDecl, Initializer, CandidateSet,
+              /*SuppressUserConversions=*/true,
+              /*PartialOverloading*/ false, AllowExplicitCtors);
       }
     }
   }
@@ -4410,17 +4414,17 @@ static OverloadingResult TryRefInitWithConversionFunction(
       // candidates with reference-compatible results? That might be needed to
       // break recursion.
       if ((AllowExplicitConvs || !Conv->isExplicit()) &&
-          (AllowRValues || Conv->getConversionType()->isLValueReferenceType())){
+          (AllowRValues ||
+           Conv->getConversionType()->isLValueReferenceType())) {
         if (ConvTemplate)
-          S.AddTemplateConversionCandidate(ConvTemplate, I.getPair(),
-                                           ActingDC, Initializer,
-                                           DestType, CandidateSet,
-                                           /*AllowObjCConversionOnExplicit=*/
-                                             false);
+          S.AddTemplateConversionCandidate(
+              ConvTemplate, I.getPair(), ActingDC, Initializer, DestType,
+              CandidateSet,
+              /*AllowObjCConversionOnExplicit=*/false, AllowExplicitConvs);
         else
-          S.AddConversionCandidate(Conv, I.getPair(), ActingDC,
-                                   Initializer, DestType, CandidateSet,
-                                   /*AllowObjCConversionOnExplicit=*/false);
+          S.AddConversionCandidate(
+              Conv, I.getPair(), ActingDC, Initializer, DestType, CandidateSet,
+              /*AllowObjCConversionOnExplicit=*/false, AllowExplicitConvs);
       }
     }
   }
@@ -4996,14 +5000,16 @@ static void TryUserDefinedConversion(Sema &S,
         if (!Info.Constructor->isInvalidDecl() &&
             Info.Constructor->isConvertingConstructor(AllowExplicit)) {
           if (Info.ConstructorTmpl)
-            S.AddTemplateOverloadCandidate(Info.ConstructorTmpl, Info.FoundDecl,
-                                           /*ExplicitArgs*/ nullptr,
-                                           Initializer, CandidateSet,
-                                           /*SuppressUserConversions=*/true);
+            S.AddTemplateOverloadCandidate(
+                Info.ConstructorTmpl, Info.FoundDecl,
+                /*ExplicitArgs*/ nullptr, Initializer, CandidateSet,
+                /*SuppressUserConversions=*/true,
+                /*PartialOverloading*/ false, AllowExplicit);
           else
             S.AddOverloadCandidate(Info.Constructor, Info.FoundDecl,
                                    Initializer, CandidateSet,
-                                   /*SuppressUserConversions=*/true);
+                                   /*SuppressUserConversions=*/true,
+                                   /*PartialOverloading*/ false, AllowExplicit);
         }
       }
     }
@@ -5038,12 +5044,12 @@ static void TryUserDefinedConversion(Sema &S,
 
         if (AllowExplicit || !Conv->isExplicit()) {
           if (ConvTemplate)
-            S.AddTemplateConversionCandidate(ConvTemplate, I.getPair(),
-                                             ActingDC, Initializer, DestType,
-                                             CandidateSet, AllowExplicit);
+            S.AddTemplateConversionCandidate(
+                ConvTemplate, I.getPair(), ActingDC, Initializer, DestType,
+                CandidateSet, AllowExplicit, AllowExplicit);
           else
-            S.AddConversionCandidate(Conv, I.getPair(), ActingDC,
-                                     Initializer, DestType, CandidateSet,
+            S.AddConversionCandidate(Conv, I.getPair(), ActingDC, Initializer,
+                                     DestType, CandidateSet, AllowExplicit,
                                      AllowExplicit);
         }
       }
@@ -9336,6 +9342,7 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
   OverloadCandidateSet::iterator Best;
 
   bool HasAnyDeductionGuide = false;
+  bool AllowExplicit = !Kind.isCopyInit() || ListInit;
 
   auto tryToResolveOverload =
       [&](bool OnlyListConstructors) -> OverloadingResult {
@@ -9361,7 +9368,7 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
       //   converting constructors (12.3.1) of that class.
       // C++ [over.match.copy]p1: (non-list copy-initialization from class)
       //   The converting constructors of T are candidate functions.
-      if (Kind.isCopyInit() && !ListInit) {
+      if (!AllowExplicit) {
         // Only consider converting constructors.
         if (GD->isExplicit())
           continue;
@@ -9396,11 +9403,13 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
 
       if (TD)
         AddTemplateOverloadCandidate(TD, I.getPair(), /*ExplicitArgs*/ nullptr,
-                                     Inits, Candidates,
-                                     SuppressUserConversions);
+                                     Inits, Candidates, SuppressUserConversions,
+                                     /*PartialOverloading*/ false,
+                                     AllowExplicit);
       else
         AddOverloadCandidate(GD, I.getPair(), Inits, Candidates,
-                             SuppressUserConversions);
+                             SuppressUserConversions,
+                             /*PartialOverloading*/ false, AllowExplicit);
     }
     return Candidates.BestViableFunction(*this, Kind.getLocation(), Best);
   };

diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
@@ -1325,7 +1325,7 @@ static void addFunctionPointerConversion(Sema &S,
   CXXConversionDecl *Conversion = CXXConversionDecl::Create(
       S.Context, Class, Loc,
       DeclarationNameInfo(ConversionName, Loc, ConvNameLoc), ConvTy, ConvTSI,
-      /*isInline=*/true, /*isExplicit=*/false,
+      /*isInline=*/true, ExplicitSpecifier(),
       /*isConstexpr=*/S.getLangOpts().CPlusPlus17,
       CallOperator->getBody()->getEndLoc());
   Conversion->setAccess(AS_public);
@@ -1412,7 +1412,7 @@ static void addBlockPointerConversion(Sema &S,
   CXXConversionDecl *Conversion = CXXConversionDecl::Create(
       S.Context, Class, Loc, DeclarationNameInfo(Name, Loc, NameLoc), ConvTy,
       S.Context.getTrivialTypeSourceInfo(ConvTy, Loc),
-      /*isInline=*/true, /*isExplicit=*/false,
+      /*isInline=*/true, ExplicitSpecifier(),
       /*isConstexpr=*/false, CallOperator->getBody()->getEndLoc());
   Conversion->setAccess(AS_public);
   Conversion->setImplicit(true);

diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
@@ -3041,10 +3041,11 @@ Sema::SpecialMemberOverloadResult Sema::LookupSpecialMember(CXXRecordDecl *RD,
                            llvm::makeArrayRef(&Arg, NumArgs), OCS, true);
       else if (CtorInfo)
         AddOverloadCandidate(CtorInfo.Constructor, CtorInfo.FoundDecl,
-                             llvm::makeArrayRef(&Arg, NumArgs), OCS, true);
+                             llvm::makeArrayRef(&Arg, NumArgs), OCS,
+                             /*SuppressUserConversions*/ true);
       else
         AddOverloadCandidate(M, Cand, llvm::makeArrayRef(&Arg, NumArgs), OCS,
-                             true);
+                             /*SuppressUserConversions*/ true);
     } else if (FunctionTemplateDecl *Tmpl =
                  dyn_cast<FunctionTemplateDecl>(Cand->getUnderlyingDecl())) {
       if (SM == CXXCopyAssignment || SM == CXXMoveAssignment)

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
@@ -1156,7 +1156,7 @@ NamedDecl *Sema::ActOnNonTypeTemplateParameter(Scope *S, Declarator &D,
     if (DS.isVirtualSpecified())
       EmitDiag(DS.getVirtualSpecLoc());
 
-    if (DS.isExplicitSpecified())
+    if (DS.hasExplicitSpecifier())
       EmitDiag(DS.getExplicitSpecLoc());
 
     if (DS.isNoreturnSpecified())
@@ -1836,8 +1836,8 @@ struct ConvertConstructorToDeductionGuideTransform {
       return nullptr;
     TypeSourceInfo *NewTInfo = TLB.getTypeSourceInfo(SemaRef.Context, NewType);
 
-    return buildDeductionGuide(TemplateParams, CD->isExplicit(), NewTInfo,
-                               CD->getBeginLoc(), CD->getLocation(),
+    return buildDeductionGuide(TemplateParams, CD->getExplicitSpecifier(),
+                               NewTInfo, CD->getBeginLoc(), CD->getLocation(),
                                CD->getEndLoc());
   }
 
@@ -1866,8 +1866,8 @@ struct ConvertConstructorToDeductionGuideTransform {
       Params.push_back(NewParam);
     }
 
-    return buildDeductionGuide(Template->getTemplateParameters(), false, TSI,
-                               Loc, Loc, Loc);
+    return buildDeductionGuide(Template->getTemplateParameters(),
+                               ExplicitSpecifier(), TSI, Loc, Loc, Loc);
   }
 
 private:
@@ -2017,7 +2017,7 @@ struct ConvertConstructorToDeductionGuideTransform {
   }
 
   NamedDecl *buildDeductionGuide(TemplateParameterList *TemplateParams,
-                                 bool Explicit, TypeSourceInfo *TInfo,
+                                 ExplicitSpecifier ES, TypeSourceInfo *TInfo,
                                  SourceLocation LocStart, SourceLocation Loc,
                                  SourceLocation LocEnd) {
     DeclarationNameInfo Name(DeductionGuideName, Loc);
@@ -2026,8 +2026,8 @@ struct ConvertConstructorToDeductionGuideTransform {
 
     // Build the implicit deduction guide template.
     auto *Guide =
-        CXXDeductionGuideDecl::Create(SemaRef.Context, DC, LocStart, Explicit,
-                                      Name, TInfo->getType(), TInfo, LocEnd);
+        CXXDeductionGuideDecl::Create(SemaRef.Context, DC, LocStart, ES, Name,
+                                      TInfo->getType(), TInfo, LocEnd);
     Guide->setImplicit();
     Guide->setParams(Params);
 

diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -366,6 +366,29 @@ static void instantiateDependentAMDGPUFlatWorkGroupSizeAttr(
                                    Attr.getSpellingListIndex());
 }
 
+static ExplicitSpecifier
+instantiateExplicitSpecifier(Sema &S,
+                             const MultiLevelTemplateArgumentList &TemplateArgs,
+                             ExplicitSpecifier ES, FunctionDecl *New) {
+  if (!ES.getExpr())
+    return ES;
+  Expr *OldCond = ES.getExpr();
+  Expr *Cond = nullptr;
+  {
+    EnterExpressionEvaluationContext Unevaluated(
+        S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+    ExprResult SubstResult = S.SubstExpr(OldCond, TemplateArgs);
+    if (SubstResult.isInvalid()) {
+      return ExplicitSpecifier::Invalid();
+    }
+    Cond = SubstResult.get();
+  }
+  ExplicitSpecifier Result(Cond, ES.getKind());
+  if (!Cond->isTypeDependent())
+    S.tryResolveExplicitSpecifier(Result);
+  return Result;
+}
+
 static void instantiateDependentAMDGPUWavesPerEUAttr(
     Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
     const AMDGPUWavesPerEUAttr &Attr, Decl *New) {
@@ -1690,6 +1713,14 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(FunctionDecl *D,
       cast<Decl>(Owner)->isDefinedOutsideFunctionOrMethod());
   LocalInstantiationScope Scope(SemaRef, MergeWithParentScope);
 
+  ExplicitSpecifier InstantiatedExplicitSpecifier;
+  if (auto *DGuide = dyn_cast<CXXDeductionGuideDecl>(D)) {
+    InstantiatedExplicitSpecifier = instantiateExplicitSpecifier(
+        SemaRef, TemplateArgs, DGuide->getExplicitSpecifier(), DGuide);
+    if (InstantiatedExplicitSpecifier.isInvalid())
+      return nullptr;
+  }
+
   SmallVector<ParmVarDecl *, 4> Params;
   TypeSourceInfo *TInfo = SubstFunctionType(D, Params);
   if (!TInfo)
@@ -1727,8 +1758,9 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(FunctionDecl *D,
   FunctionDecl *Function;
   if (auto *DGuide = dyn_cast<CXXDeductionGuideDecl>(D)) {
     Function = CXXDeductionGuideDecl::Create(
-      SemaRef.Context, DC, D->getInnerLocStart(), DGuide->isExplicit(),
-      NameInfo, T, TInfo, D->getSourceRange().getEnd());
+        SemaRef.Context, DC, D->getInnerLocStart(),
+        InstantiatedExplicitSpecifier, NameInfo, T, TInfo,
+        D->getSourceRange().getEnd());
     if (DGuide->isCopyDeductionCandidate())
       cast<CXXDeductionGuideDecl>(Function)->setIsCopyDeductionCandidate();
     Function->setAccess(D->getAccess());
@@ -1996,6 +2028,12 @@ Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
     }
   }
 
+  ExplicitSpecifier InstantiatedExplicitSpecifier =
+      instantiateExplicitSpecifier(SemaRef, TemplateArgs,
+                                   ExplicitSpecifier::getFromDecl(D), D);
+  if (InstantiatedExplicitSpecifier.isInvalid())
+    return nullptr;
+
   SmallVector<ParmVarDecl *, 4> Params;
   TypeSourceInfo *TInfo = SubstFunctionType(D, Params);
   if (!TInfo)
@@ -2035,11 +2073,10 @@ Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
   DeclarationNameInfo NameInfo
     = SemaRef.SubstDeclarationNameInfo(D->getNameInfo(), TemplateArgs);
   if (CXXConstructorDecl *Constructor = dyn_cast<CXXConstructorDecl>(D)) {
-    Method = CXXConstructorDecl::Create(SemaRef.Context, Record,
-                                        StartLoc, NameInfo, T, TInfo,
-                                        Constructor->isExplicit(),
-                                        Constructor->isInlineSpecified(),
-                                        false, Constructor->isConstexpr());
+    Method = CXXConstructorDecl::Create(
+        SemaRef.Context, Record, StartLoc, NameInfo, T, TInfo,
+        InstantiatedExplicitSpecifier, Constructor->isInlineSpecified(), false,
+        Constructor->isConstexpr());
     Method->setRangeEnd(Constructor->getEndLoc());
   } else if (CXXDestructorDecl *Destructor = dyn_cast<CXXDestructorDecl>(D)) {
     Method = CXXDestructorDecl::Create(SemaRef.Context, Record,
@@ -2050,7 +2087,7 @@ Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
   } else if (CXXConversionDecl *Conversion = dyn_cast<CXXConversionDecl>(D)) {
     Method = CXXConversionDecl::Create(
         SemaRef.Context, Record, StartLoc, NameInfo, T, TInfo,
-        Conversion->isInlineSpecified(), Conversion->isExplicit(),
+        Conversion->isInlineSpecified(), InstantiatedExplicitSpecifier,
         Conversion->isConstexpr(), Conversion->getEndLoc());
   } else {
     StorageClass SC = D->isStatic() ? SC_Static : SC_None;

diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -858,7 +858,6 @@ void ASTDeclReader::VisitFunctionDecl(FunctionDecl *FD) {
   FD->setStorageClass(static_cast<StorageClass>(Record.readInt()));
   FD->setInlineSpecified(Record.readInt());
   FD->setImplicitlyInline(Record.readInt());
-  FD->setExplicitSpecified(Record.readInt());
   FD->setVirtualAsWritten(Record.readInt());
   FD->setPure(Record.readInt());
   FD->setHasInheritedPrototype(Record.readInt());
@@ -1977,6 +1976,7 @@ ASTDeclReader::VisitCXXRecordDeclImpl(CXXRecordDecl *D) {
 }
 
 void ASTDeclReader::VisitCXXDeductionGuideDecl(CXXDeductionGuideDecl *D) {
+  D->setExplicitSpecifier(Record.readExplicitSpec());
   VisitFunctionDecl(D);
   D->setIsCopyDeductionCandidate(Record.readInt());
 }
@@ -2002,6 +2002,7 @@ void ASTDeclReader::VisitCXXMethodDecl(CXXMethodDecl *D) {
 void ASTDeclReader::VisitCXXConstructorDecl(CXXConstructorDecl *D) {
   // We need the inherited constructor information to merge the declaration,
   // so we have to read it before we call VisitCXXMethodDecl.
+  D->setExplicitSpecifier(Record.readExplicitSpec());
   if (D->isInheritingConstructor()) {
     auto *Shadow = ReadDeclAs<ConstructorUsingShadowDecl>();
     auto *Ctor = ReadDeclAs<CXXConstructorDecl>();
@@ -2027,6 +2028,7 @@ void ASTDeclReader::VisitCXXDestructorDecl(CXXDestructorDecl *D) {
 }
 
 void ASTDeclReader::VisitCXXConversionDecl(CXXConversionDecl *D) {
+  D->setExplicitSpecifier(Record.readExplicitSpec());
   VisitCXXMethodDecl(D);
 }
 
@@ -3750,10 +3752,7 @@ Decl *ASTReader::ReadDeclRecord(DeclID ID) {
     D = CXXMethodDecl::CreateDeserialized(Context, ID);
     break;
   case DECL_CXX_CONSTRUCTOR:
-    D = CXXConstructorDecl::CreateDeserialized(Context, ID, false);
-    break;
-  case DECL_CXX_INHERITED_CONSTRUCTOR:
-    D = CXXConstructorDecl::CreateDeserialized(Context, ID, true);
+    D = CXXConstructorDecl::CreateDeserialized(Context, ID, Record.readInt());
     break;
   case DECL_CXX_DESTRUCTOR:
     D = CXXDestructorDecl::CreateDeserialized(Context, ID);

diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
@@ -1277,7 +1277,6 @@ void ASTWriter::WriteBlockInfoBlock() {
   RECORD(DECL_CXX_RECORD);
   RECORD(DECL_CXX_METHOD);
   RECORD(DECL_CXX_CONSTRUCTOR);
-  RECORD(DECL_CXX_INHERITED_CONSTRUCTOR);
   RECORD(DECL_CXX_DESTRUCTOR);
   RECORD(DECL_CXX_CONVERSION);
   RECORD(DECL_ACCESS_SPEC);

diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -535,7 +535,6 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
   Record.push_back(static_cast<int>(D->getStorageClass())); // FIXME: stable encoding
   Record.push_back(D->isInlineSpecified());
   Record.push_back(D->isInlined());
-  Record.push_back(D->isExplicitSpecified());
   Record.push_back(D->isVirtualAsWritten());
   Record.push_back(D->isPure());
   Record.push_back(D->hasInheritedPrototype());
@@ -638,7 +637,18 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
   Code = serialization::DECL_FUNCTION;
 }
 
+static void addExplicitSpecifier(ExplicitSpecifier ES,
+                                 ASTRecordWriter &Record) {
+  uint64_t Kind = static_cast<uint64_t>(ES.getKind());
+  Kind = Kind << 1 | static_cast<bool>(ES.getExpr());
+  Record.push_back(Kind);
+  if (ES.getExpr()) {
+    Record.AddStmt(ES.getExpr());
+  }
+}
+
 void ASTDeclWriter::VisitCXXDeductionGuideDecl(CXXDeductionGuideDecl *D) {
+  addExplicitSpecifier(D->getExplicitSpecifier(), Record);
   VisitFunctionDecl(D);
   Record.push_back(D->isCopyDeductionCandidate());
   Code = serialization::DECL_CXX_DEDUCTION_GUIDE;
@@ -1331,19 +1341,15 @@ void ASTDeclWriter::VisitCXXMethodDecl(CXXMethodDecl *D) {
 }
 
 void ASTDeclWriter::VisitCXXConstructorDecl(CXXConstructorDecl *D) {
+  Record.push_back(D->getTraillingAllocKind());
+  addExplicitSpecifier(D->getExplicitSpecifier(), Record);
   if (auto Inherited = D->getInheritedConstructor()) {
     Record.AddDeclRef(Inherited.getShadowDecl());
     Record.AddDeclRef(Inherited.getConstructor());
-    Code = serialization::DECL_CXX_INHERITED_CONSTRUCTOR;
-  } else {
-    Code = serialization::DECL_CXX_CONSTRUCTOR;
   }
 
   VisitCXXMethodDecl(D);
-
-  Code = D->isInheritingConstructor()
-             ? serialization::DECL_CXX_INHERITED_CONSTRUCTOR
-             : serialization::DECL_CXX_CONSTRUCTOR;
+  Code = serialization::DECL_CXX_CONSTRUCTOR;
 }
 
 void ASTDeclWriter::VisitCXXDestructorDecl(CXXDestructorDecl *D) {
@@ -1357,6 +1363,7 @@ void ASTDeclWriter::VisitCXXDestructorDecl(CXXDestructorDecl *D) {
 }
 
 void ASTDeclWriter::VisitCXXConversionDecl(CXXConversionDecl *D) {
+  addExplicitSpecifier(D->getExplicitSpecifier(), Record);
   VisitCXXMethodDecl(D);
   Code = serialization::DECL_CXX_CONVERSION;
 }
@@ -2156,7 +2163,6 @@ void ASTWriter::WriteDeclAbbrevs() {
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 3)); // StorageClass
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // Inline
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // InlineSpecified
-  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // ExplicitSpecified
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // VirtualAsWritten
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // Pure
   Abv->Add(BitCodeAbbrevOp(0));                         // HasInheritedProto

diff --git a/clang/test/CXX/temp/temp.deduct.guide/p1.cpp b/clang/test/CXX/temp/temp.deduct.guide/p1.cpp
@@ -71,7 +71,7 @@ extern A(int(&)[26]) -> A<int>;
 #endif
 mutable A(int(&)[27]) -> A<int>; // expected-error-re {{{{'mutable' cannot be applied to|illegal storage class on}} function}}
 virtual A(int(&)[28]) -> A<int>; // expected-error {{'virtual' can only appear on non-static member functions}}
-const A(int(&)[28]) -> A<int>; // expected-error {{deduction guide cannot be declared 'const'}}
+const A(int(&)[31]) -> A<int>; // expected-error {{deduction guide cannot be declared 'const'}}
 
 const volatile static constexpr inline A(int(&)[29]) -> A<int>; // expected-error {{deduction guide cannot be declared 'static inline constexpr const volatile'}}
 

diff --git a/clang/test/CXX/temp/temp.deduct.guide/p3.cpp b/clang/test/CXX/temp/temp.deduct.guide/p3.cpp
@@ -3,14 +3,14 @@
 // The same restrictions apply to the parameter-declaration-clause of a
 // deduction guide as in a function declaration.
 template<typename T> struct A {};
-A(void) -> A<int>; // ok
+A(void) -> A<int>; // expected-note {{previous}}
 A(void, int) -> A<int>; // expected-error {{'void' must be the first and only parameter if specified}}
 
-// We interpret this as also extending to the validity of redeclarations. It's
-// a bit of a stretch (OK, a lot of a stretch) but it gives desirable answers.
-A() -> A<int>; // ok, redeclaration
+A() -> A<int>; // expected-error {{redeclaration of deduction guide}}
+// expected-note@-1 {{previous}}
 
 A() -> A<int>; // expected-note {{previous}}
+// expected-error@-1 {{redeclaration of deduction guide}}
 A() -> A<float>; // FIXME: "functions" is a poor term. expected-error {{functions that differ only in their return type cannot be overloaded}}
 
 template<typename T> A(T) -> A<typename T::foo>;

diff --git a/clang/test/PCH/cxx-explicit-specifier.cpp b/clang/test/PCH/cxx-explicit-specifier.cpp
@@ -0,0 +1,124 @@
+// RUN: %clang_cc1 -std=c++2a -emit-pch %s -o %t-cxx2a
+// RUN: %clang_cc1 -std=c++2a -DUSE_PCH -include-pch %t-cxx2a %s -ast-print -verify | FileCheck %s
+
+#ifndef USE_PCH
+namespace inheriting_constructor {
+  struct S {};
+
+  template<typename X, typename Y> struct T {
+    template<typename A>
+    explicit((Y{}, true)) T(A &&a) {}
+  };
+
+  template<typename X, typename Y> struct U : T<X, Y> {
+    using T<X, Y>::T;
+  };
+
+  U<S, char> foo(char ch) {
+    return U<S, char>(ch);
+  }
+}
+#else
+namespace inheriting_constructor {
+U<S, char> a = foo('0');
+}
+
+//CHECK: explicit((char{} , true))
+
+#endif
+
+namespace basic {
+#ifndef USE_PCH
+
+struct B {};
+
+struct A {
+  explicit A(int);
+  explicit(false) operator bool();
+  explicit(true) operator B();
+};
+#else
+//expected-note@-6+ {{candidate constructor}}
+//expected-note@-9+ {{candidate constructor}}
+//expected-note@-6+ {{candidate function}}
+
+//CHECK: explicit{{ +}}A(
+//CHECK-NEXT: explicit(false){{ +}}operator
+//CHECK-NEXT: explicit(true){{ +}}operator
+A a = 0; //expected-error {{no viable conversion}}
+A a1(0);
+
+bool b = a1;
+B b1 = a1; //expected-error {{no viable conversion}}
+
+#endif
+}
+
+
+namespace templ {
+#ifndef USE_PCH
+
+template<bool b>
+struct B {
+  static constexpr bool value = b;
+};
+
+template<bool b>
+struct A {
+  explicit(b) A(B<b>) {}
+  template<typename T>
+  explicit(b ^ T::value) operator T();
+};
+B<true> b_true;
+B<false> b_false;
+#else
+//expected-note@-8 {{candidate template ignored}}
+//expected-note@-8+ {{explicit constructor}}
+//expected-note@-15+ {{candidate constructor}}
+//expected-note@-8+ {{candidate conversion operator ignored}}
+//expected-note@-9+ {{explicit(bool) specifier resolved to true}}
+//expected-note@-12 {{explicit(bool) specifier resolved to true}}
+//expected-note@-13+ {{candidate deductiong guide ignored}}
+
+//CHECK: explicit(b){{ +}}A
+//CHECK: explicit(b{{ +}}^{{ +}}T::value){{ +}}operator
+
+A a = { b_true }; //expected-error {{class template argument deduction}}
+A a0 = b_true; //expected-error {{no viable constructor or deduction guide}}
+A a_true(b_true);
+A a_false = b_false;
+
+B<true> b = a_true;
+B<true> b1 = a_false; //expected-error {{no viable conversion}}
+B<false> b2(a_true);
+
+#endif
+
+}
+
+namespace guide {
+
+#ifndef USE_PCH
+
+template<typename T>
+struct A {
+  A(T);
+};
+
+template<typename T>
+explicit(true) A(T) -> A<T>;
+
+explicit(false) A(int) -> A<int>;
+
+#else
+//expected-note@-5 {{explicit deduction guide}}
+
+//CHECK: explicit(true){{ +}}A(
+//CHECK: explicit(false){{ +}}A(
+
+A a = { 0.0 }; //expected-error {{explicit deduction guide}}
+A a1 = { 0 };
+
+#endif
+
+}
diff --git a/clang/test/SemaCXX/builtin-is-constant-evaluated.cpp b/clang/test/SemaCXX/builtin-is-constant-evaluated.cpp
@@ -115,7 +115,7 @@ static_assert(&r == &x);
 
 #if defined(__cpp_conditional_explicit)
 struct TestConditionalExplicit {
-  explicit(__builtin_is_constant_evaluated()) TestConditionalExplicit(int) {}
+  explicit(!__builtin_is_constant_evaluated()) TestConditionalExplicit(int) {}
 };
 TestConditionalExplicit e = 42;
 #endif
diff --git a/clang/test/SemaCXX/cxx2a-compat.cpp b/clang/test/SemaCXX/cxx2a-compat.cpp
@@ -37,3 +37,23 @@ string u8str = u8"test" u8"test";
 // expected-error@-8 {{cannot initialize a variable of type 'const char *' with an lvalue of type 'const char8_t [6]'}}
 // expected-error@-8 {{no viable conversion from 'const char8_t [9]' to 'string'}}
 #endif
+
+template<bool b>
+struct C {
+  explicit(C)(int);
+};
+#if __cplusplus <= 201703L
+// expected-warning@-3 {{this expression will be parsed as explicit(bool) in C++2a}}
+#if defined(__cpp_conditional_explicit)
+#error "the feature test macro __cpp_conditional_explicit isn't correct"
+#endif
+#else
+// expected-error@-8 {{does not refer to a value}}
+// expected-error@-9 {{expected member name or ';'}}
+// expected-error@-10 {{expected ')'}}
+// expected-note@-12 {{declared here}}
+// expected-note@-12 {{to match this '('}}
+#if !defined(__cpp_conditional_explicit) || __cpp_conditional_explicit != 201806L
+#error "the feature test macro __cpp_conditional_explicit isn't correct"
+#endif
+#endif
diff --git a/clang/test/SemaCXX/cxx2a-explicit-bool.cpp b/clang/test/SemaCXX/cxx2a-explicit-bool.cpp
diff --git a/clang/test/SemaCXX/explicit.cpp b/clang/test/SemaCXX/explicit.cpp
@@ -1,4 +1,6 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++2a %s
+
 namespace Constructor {
 struct A {
   A(int);
@@ -34,6 +36,21 @@ B &&b3(0); // expected-error {{could not bind}}
 B b4{0};
 B &&b5 = {0}; // expected-error {{chosen constructor is explicit}}
 B &&b6{0};
+
+struct S {
+  template <bool b = true>
+  explicit S();
+};
+
+struct T : S {
+  //  T();
+};
+
+struct U : T {
+  U();
+};
+U::U() {}
+
 }
 
 namespace Conversion {
@@ -183,7 +200,8 @@ namespace Conversion {
     const int &copyList7 = {b};
     const int &copyList8 = {n}; // expected-error {{no viable conversion}}
   }
-
+
+#if __cplusplus < 201707L
   void testNew()
   {
     // 5.3.4p6:
@@ -200,7 +218,8 @@ namespace Conversion {
     new int[i];
     new int[ni]; // expected-error {{array size expression of type 'NotInt' requires explicit conversion to type 'int'}}
   }
-
+#endif
+
   void testDelete()
   {
     // 5.3.5pp2:

diff --git a/clang/unittests/AST/Language.cpp b/clang/unittests/AST/Language.cpp
@@ -34,6 +34,9 @@ ArgVector getBasicRunOptionsForLanguage(Language Lang) {
   case Lang_CXX14:
     BasicArgs = {"-std=c++14", "-frtti"};
     break;
+  case Lang_CXX2a:
+    BasicArgs = {"-std=c++2a", "-frtti"};
+    break;
   case Lang_OpenCL:
   case Lang_OBJCXX:
     llvm_unreachable("Not implemented yet!");

diff --git a/clang/unittests/AST/Language.h b/clang/unittests/AST/Language.h
@@ -28,6 +28,7 @@ enum Language {
     Lang_CXX,
     Lang_CXX11,
     Lang_CXX14,
+    Lang_CXX2a,
     Lang_OpenCL,
     Lang_OBJCXX
 };

diff --git a/clang/unittests/AST/MatchVerifier.h b/clang/unittests/AST/MatchVerifier.h
@@ -108,6 +108,10 @@ testing::AssertionResult MatchVerifier<NodeType>::match(
     Args.push_back("-std=c++14");
     FileName = "input.cc";
     break;
+  case Lang_CXX2a:
+    Args.push_back("-std=c++2a");
+    FileName = "input.cc";
+    break;
   case Lang_OpenCL:
     FileName = "input.cl";
     break;

diff --git a/clang/unittests/AST/StructuralEquivalenceTest.cpp b/clang/unittests/AST/StructuralEquivalenceTest.cpp
@@ -806,6 +806,26 @@ TEST_F(StructuralEquivalenceTest, CompareSameDeclWithMultiple) {
   EXPECT_FALSE(testStructuralMatch(t));
 }
 
+TEST_F(StructuralEquivalenceTest, ExplicitBoolDifferent) {
+  auto Decls = makeNamedDecls("struct foo {explicit(false) foo(int);};",
+                              "struct foo {explicit(true) foo(int);};", Lang_CXX2a);
+  CXXConstructorDecl *First = FirstDeclMatcher<CXXConstructorDecl>().match(
+      get<0>(Decls), cxxConstructorDecl(hasName("foo")));
+  CXXConstructorDecl *Second = FirstDeclMatcher<CXXConstructorDecl>().match(
+      get<1>(Decls), cxxConstructorDecl(hasName("foo")));
+  EXPECT_FALSE(testStructuralMatch(First, Second));
+}
+
+TEST_F(StructuralEquivalenceTest, ExplicitBoolSame) {
+  auto Decls = makeNamedDecls("struct foo {explicit(true) foo(int);};",
+                              "struct foo {explicit(true) foo(int);};", Lang_CXX2a);
+  CXXConstructorDecl *First = FirstDeclMatcher<CXXConstructorDecl>().match(
+      get<0>(Decls), cxxConstructorDecl(hasName("foo")));
+  CXXConstructorDecl *Second = FirstDeclMatcher<CXXConstructorDecl>().match(
+      get<1>(Decls), cxxConstructorDecl(hasName("foo")));
+  EXPECT_TRUE(testStructuralMatch(First, Second));
+}
+
 struct StructuralEquivalenceEnumTest : StructuralEquivalenceTest {};
 
 TEST_F(StructuralEquivalenceEnumTest, FwdDeclEnumShouldBeEqualWithFwdDeclEnum) {
@@ -853,5 +873,25 @@ TEST_F(StructuralEquivalenceTemplateTest, DifferentTemplateArgKind) {
   EXPECT_FALSE(testStructuralMatch(t));
 }
 
+TEST_F(StructuralEquivalenceTemplateTest, ExplicitBoolSame) {
+  auto Decls = makeNamedDecls("template <bool b> struct foo {explicit(b) foo(int);};",
+                              "template <bool b> struct foo {explicit(b) foo(int);};", Lang_CXX2a);
+  CXXConstructorDecl *First = FirstDeclMatcher<CXXConstructorDecl>().match(
+      get<0>(Decls), cxxConstructorDecl(hasName("foo<b>")));
+  CXXConstructorDecl *Second = FirstDeclMatcher<CXXConstructorDecl>().match(
+      get<1>(Decls), cxxConstructorDecl(hasName("foo<b>")));
+  EXPECT_TRUE(testStructuralMatch(First, Second));
+}
+
+TEST_F(StructuralEquivalenceTemplateTest, ExplicitBoolDifference) {
+  auto Decls = makeNamedDecls("template <bool b> struct foo {explicit(b) foo(int);};",
+                              "template <bool b> struct foo {explicit(!b) foo(int);};", Lang_CXX2a);
+  CXXConstructorDecl *First = FirstDeclMatcher<CXXConstructorDecl>().match(
+      get<0>(Decls), cxxConstructorDecl(hasName("foo<b>")));
+  CXXConstructorDecl *Second = FirstDeclMatcher<CXXConstructorDecl>().match(
+      get<1>(Decls), cxxConstructorDecl(hasName("foo<b>")));
+  EXPECT_FALSE(testStructuralMatch(First, Second));
+}
+
 } // end namespace ast_matchers
 } // end namespace clang
diff --git a/clang/www/cxx_status.html b/clang/www/cxx_status.html
@@ -1002,7 +1002,7 @@ <h2 id="cxx20">C++2a implementation status</h2>
     <tr>
       <td><tt>explicit(bool)</tt></td>
       <td><a href="http://wg21.link/p0892r2">P0892R2</a></td>
-      <td class="none" align="center">No</td>
+      <td class="svn" align="center">SVN</td>
     </tr>
     <!-- San Diego papers -->
     <tr>