Revert "Revert "[Concepts] Fix overload resolution bug with constrain…

…ed candidates""

This reverts commit f6af446.

clang/docs/ReleaseNotes.rst

@@ -123,6 +123,11 @@ Bug Fixes
   a lambda expression that shares the name of a variable in a containing
   if/while/for/switch init statement as a redeclaration.
   This fixes `Issue 54913 <https://github.com/llvm/llvm-project/issues/54913>`_.
+- Overload resolution for constrained function templates could use the partial
+  order of constraints to select an overload, even if the parameter types of
+  the functions were different. It now diagnoses this case correctly as an
+  ambiguous call and an error. Fixes
+  `Issue 53640 <https://github.com/llvm/llvm-project/issues/53640>`_.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -3580,7 +3580,8 @@ class Sema final {
                                 QualType& ConvertedType);
   bool FunctionParamTypesAreEqual(const FunctionProtoType *OldType,
                                   const FunctionProtoType *NewType,
-                                  unsigned *ArgPos = nullptr);
+                                  unsigned *ArgPos = nullptr,
+                                  bool Reversed = false);
   void HandleFunctionTypeMismatch(PartialDiagnostic &PDiag,
                                   QualType FromType, QualType ToType);
 
@@ -8749,7 +8750,8 @@ class Sema final {
   FunctionTemplateDecl *getMoreSpecializedTemplate(
       FunctionTemplateDecl *FT1, FunctionTemplateDecl *FT2, SourceLocation Loc,
       TemplatePartialOrderingContext TPOC, unsigned NumCallArguments1,
-      unsigned NumCallArguments2, bool Reversed = false);
+      unsigned NumCallArguments2, bool Reversed = false,
+      bool AllowOrderingByConstraints = true);
   UnresolvedSetIterator
   getMostSpecialized(UnresolvedSetIterator SBegin, UnresolvedSetIterator SEnd,
                      TemplateSpecCandidateSet &FailedCandidates,

clang/lib/Sema/SemaOverload.cpp

@@ -2945,24 +2945,30 @@ void Sema::HandleFunctionTypeMismatch(PartialDiagnostic &PDiag,
 }
 
 /// FunctionParamTypesAreEqual - This routine checks two function proto types
-/// for equality of their argument types. Caller has already checked that
-/// they have same number of arguments.  If the parameters are different,
+/// 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) {
-  for (FunctionProtoType::param_type_iterator O = OldType->param_type_begin(),
-                                              N = NewType->param_type_begin(),
-                                              E = OldType->param_type_end();
-       O && (O != E); ++O, ++N) {
+                                      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(O->getUnqualifiedType());
-    QualType New = Context.removePtrSizeAddrSpace(N->getUnqualifiedType());
+    QualType Old = Context.removePtrSizeAddrSpace(OldType->getParamType(I).getUnqualifiedType());
+    QualType New = Context.removePtrSizeAddrSpace(NewType->getParamType(J).getUnqualifiedType());
 
     if (!Context.hasSameType(Old, New)) {
       if (ArgPos)
-        *ArgPos = O - OldType->param_type_begin();
+        *ArgPos = I;
       return false;
     }
   }
@@ -9584,6 +9590,32 @@ static bool haveSameParameterTypes(ASTContext &Context, const FunctionDecl *F1,
   return true;
 }
 
+/// We're allowed to use constraints partial ordering only if the candidates
+/// have the same parameter types:
+/// [temp.func.order]p6.2.2 [...] or if the function parameters that
+/// positionally correspond between the two templates are not of the same type,
+/// neither template is more specialized than the other.
+/// [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 canCompareFunctionConstraints(Sema &S,
+                                          const OverloadCandidate &Cand1,
+                                          const OverloadCandidate &Cand2) {
+  // FIXME: Per P2113R0 we also need to compare the template parameter lists
+  // when comparing template functions. 
+  if (Cand1.Function && Cand2.Function && Cand1.Function->hasPrototype() &&
+      Cand2.Function->hasPrototype()) {
+    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(
@@ -9815,34 +9847,28 @@ bool clang::isBetterOverloadCandidate(
             isa<CXXConversionDecl>(Cand1.Function) ? TPOC_Conversion
                                                    : TPOC_Call,
             Cand1.ExplicitCallArguments, Cand2.ExplicitCallArguments,
-            Cand1.isReversed() ^ Cand2.isReversed()))
+            Cand1.isReversed() ^ Cand2.isReversed(),
+            canCompareFunctionConstraints(S, Cand1, Cand2)))
       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 (Cand1.Function && Cand2.Function && !Cand1IsSpecialization &&
-      !Cand2IsSpecialization && Cand1.Function->hasPrototype() &&
-      Cand2.Function->hasPrototype()) {
-    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)) {
-      Expr *RC1 = Cand1.Function->getTrailingRequiresClause();
-      Expr *RC2 = Cand2.Function->getTrailingRequiresClause();
-      if (RC1 && RC2) {
-        bool AtLeastAsConstrained1, AtLeastAsConstrained2;
-        if (S.IsAtLeastAsConstrained(Cand1.Function, {RC1}, Cand2.Function,
-                                     {RC2}, AtLeastAsConstrained1) ||
-            S.IsAtLeastAsConstrained(Cand2.Function, {RC2}, Cand1.Function,
-                                     {RC1}, AtLeastAsConstrained2))
-          return false;
-        if (AtLeastAsConstrained1 != AtLeastAsConstrained2)
-          return AtLeastAsConstrained1;
-      } else if (RC1 || RC2) {
-        return RC1 != nullptr;
-      }
+  if (!Cand1IsSpecialization && !Cand2IsSpecialization &&
+      canCompareFunctionConstraints(S, Cand1, Cand2)) {
+    Expr *RC1 = Cand1.Function->getTrailingRequiresClause();
+    Expr *RC2 = Cand2.Function->getTrailingRequiresClause();
+    if (RC1 && RC2) {
+      bool AtLeastAsConstrained1, AtLeastAsConstrained2;
+      if (S.IsAtLeastAsConstrained(Cand1.Function, {RC1}, Cand2.Function, {RC2},
+                                   AtLeastAsConstrained1) ||
+          S.IsAtLeastAsConstrained(Cand2.Function, {RC2}, Cand1.Function, {RC1},
+                                   AtLeastAsConstrained2))
+        return false;
+      if (AtLeastAsConstrained1 != AtLeastAsConstrained2)
+        return AtLeastAsConstrained1;
+    } else if (RC1 || RC2) {
+      return RC1 != nullptr;
     }
   }
 

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -5143,18 +5143,20 @@ static bool isVariadicFunctionTemplate(FunctionTemplateDecl *FunTmpl) {
 /// candidate with a reversed parameter order. In this case, the corresponding
 /// P/A pairs between FT1 and FT2 are reversed.
 ///
+/// \param AllowOrderingByConstraints If \c is false, don't check whether one
+/// of the templates is more constrained than the other. Default is true.
+///
 /// \returns the more specialized function template. If neither
 /// template is more specialized, returns NULL.
-FunctionTemplateDecl *
-Sema::getMoreSpecializedTemplate(FunctionTemplateDecl *FT1,
-                                 FunctionTemplateDecl *FT2,
-                                 SourceLocation Loc,
-                                 TemplatePartialOrderingContext TPOC,
-                                 unsigned NumCallArguments1,
-                                 unsigned NumCallArguments2,
-                                 bool Reversed) {
-
-  auto JudgeByConstraints = [&] () -> FunctionTemplateDecl * {
+FunctionTemplateDecl *Sema::getMoreSpecializedTemplate(
+    FunctionTemplateDecl *FT1, FunctionTemplateDecl *FT2, SourceLocation Loc,
+    TemplatePartialOrderingContext TPOC, unsigned NumCallArguments1,
+    unsigned NumCallArguments2, bool Reversed,
+    bool AllowOrderingByConstraints) {
+
+  auto JudgeByConstraints = [&]() -> FunctionTemplateDecl * {
+    if (!AllowOrderingByConstraints)
+      return nullptr;
     llvm::SmallVector<const Expr *, 3> AC1, AC2;
     FT1->getAssociatedConstraints(AC1);
     FT2->getAssociatedConstraints(AC2);

clang/test/CXX/temp/temp.decls/temp.fct/temp.func.order/p6.cpp

@@ -0,0 +1,49 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+
+struct A;
+struct B;
+
+template <typename> constexpr bool True = true;
+template <typename T> concept C = True<T>;
+
+void f(C auto &, auto &) = delete;
+template <C Q> void f(Q &, C auto &);
+
+void g(struct A *ap, struct B *bp) {
+  f(*ap, *bp);
+}
+
+template <typename T, typename U> struct X {};
+
+template <typename T, C U, typename V> bool operator==(X<T, U>, V) = delete;
+template <C T, C U, C V>               bool operator==(T, X<U, V>);
+
+bool h() {
+  return X<void *, int>{} == 0;
+}
+
+namespace PR53640 {
+
+template <typename T>
+concept C = true;
+
+template <C T>
+void f(T t) {} // expected-note {{candidate function [with T = int]}}
+
+template <typename T>
+void f(const T &t) {} // expected-note {{candidate function [with T = int]}}
+
+int g() {
+  f(0); // expected-error {{call to 'f' is ambiguous}}
+}
+
+struct S {
+  template <typename T> explicit S(T) noexcept requires C<T> {} // expected-note {{candidate constructor}}
+  template <typename T> explicit S(const T &) noexcept {}       // expected-note {{candidate constructor}}
+};
+
+int h() {
+  S s(4); // expected-error-re {{call to constructor of {{.*}} is ambiguous}}
+}
+
+}