[Clang][Sema] Improve support for explicit specializations of constrained member functions & member function templates

clang/docs/ReleaseNotes.rst

@@ -695,6 +695,10 @@ Bug Fixes to C++ Support
   until the noexcept-specifier is instantiated.
 - Fix a crash when an implicitly declared ``operator==`` function with a trailing requires-clause has its
   constraints compared to that of another declaration.
+- Fix a bug where explicit specializations of member functions/function templates would have substitution
+  performed incorrectly when checking constraints. Fixes (#GH90349).
+- Clang now allows constrained member functions to be explicitly specialized for an implicit instantiation
+  of a class template.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5437,6 +5437,11 @@ def note_function_template_spec_matched : Note<
 def err_function_template_partial_spec : Error<
     "function template partial specialization is not allowed">;
 
+def err_function_member_spec_ambiguous : Error<
+    "ambiguous member function specialization %q0 of %q1">;
+def note_function_member_spec_matched : Note<
+    "member function specialization matches %0">;
+
 // C++ Template Instantiation
 def err_template_recursion_depth_exceeded : Error<
   "recursive template instantiation exceeded maximum depth of %0">,

clang/include/clang/Sema/Sema.h

@@ -9739,6 +9739,9 @@ class Sema final : public SemaBase {
                      const PartialDiagnostic &CandidateDiag,
                      bool Complain = true, QualType TargetType = QualType());
 
+  FunctionDecl *getMoreConstrainedFunction(FunctionDecl *FD1,
+                                           FunctionDecl *FD2);
+
   ///@}
 
   //

clang/lib/Sema/SemaConcept.cpp

@@ -811,7 +811,7 @@ static const Expr *SubstituteConstraintExpressionWithoutSatisfaction(
   // this may happen while we're comparing two templates' constraint
   // equivalence.
   LocalInstantiationScope ScopeForParameters(S);
-  if (auto *FD = llvm::dyn_cast<FunctionDecl>(DeclInfo.getDecl()))
+  if (auto *FD = DeclInfo.getDecl()->getAsFunction())
     for (auto *PVD : FD->parameters())
       ScopeForParameters.InstantiatedLocal(PVD, PVD);
 

clang/lib/Sema/SemaOverload.cpp

@@ -1303,6 +1303,8 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
   if (New->isMSVCRTEntryPoint())
     return false;
 
+  NamedDecl *OldDecl = Old;
+  NamedDecl *NewDecl = New;
   FunctionTemplateDecl *OldTemplate = Old->getDescribedFunctionTemplate();
   FunctionTemplateDecl *NewTemplate = New->getDescribedFunctionTemplate();
 
@@ -1347,6 +1349,8 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
   // references to non-instantiated entities during constraint substitution.
   // GH78101.
   if (NewTemplate) {
+    OldDecl = OldTemplate;
+    NewDecl = 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
@@ -1506,13 +1510,14 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
     }
   }
 
-  if (!UseOverrideRules) {
+  if (!UseOverrideRules &&
+      New->getTemplateSpecializationKind() != TSK_ExplicitSpecialization) {
     Expr *NewRC = New->getTrailingRequiresClause(),
          *OldRC = Old->getTrailingRequiresClause();
     if ((NewRC != nullptr) != (OldRC != nullptr))
       return true;
-
-    if (NewRC && !SemaRef.AreConstraintExpressionsEqual(Old, OldRC, New, NewRC))
+    if (NewRC &&
+        !SemaRef.AreConstraintExpressionsEqual(OldDecl, OldRC, NewDecl, NewRC))
       return true;
   }
 
@@ -10695,29 +10700,10 @@ bool clang::isBetterOverloadCandidate(
   //   -— 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;
-    }
-  }
+      sameFunctionParameterTypeLists(S, Cand1, Cand2) &&
+      S.getMoreConstrainedFunction(Cand1.Function, Cand2.Function) ==
+          Cand1.Function)
+    return true;
 
   //   -- 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
@@ -13385,25 +13371,6 @@ Sema::resolveAddressOfSingleOverloadCandidate(Expr *E, DeclAccessPair &Pair) {
            static_cast<int>(CUDA().IdentifyPreference(Caller, FD2));
   };
 
-  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/...
@@ -13440,15 +13407,14 @@ Sema::resolveAddressOfSingleOverloadCandidate(Expr *E, DeclAccessPair &Pair) {
       }
       // FD has the same CUDA prefernece than Result. Continue check
       // constraints.
-      std::optional<bool> MoreConstrainedThanPrevious =
-          CheckMoreConstrained(FD, Result);
-      if (!MoreConstrainedThanPrevious) {
-        IsResultAmbiguous = true;
-        AmbiguousDecls.push_back(FD);
+      FunctionDecl *MoreConstrained = getMoreConstrainedFunction(FD, Result);
+      if (MoreConstrained != FD) {
+        if (!MoreConstrained) {
+          IsResultAmbiguous = true;
+          AmbiguousDecls.push_back(FD);
+        }
         continue;
       }
-      if (!*MoreConstrainedThanPrevious)
-        continue;
       // FD is more constrained - replace Result with it.
     }
     FoundBetter();
@@ -13467,7 +13433,7 @@ Sema::resolveAddressOfSingleOverloadCandidate(Expr *E, DeclAccessPair &Pair) {
       // constraints.
       if (getLangOpts().CUDA && CheckCUDAPreference(Skipped, Result) != 0)
         continue;
-      if (!CheckMoreConstrained(Skipped, Result))
+      if (!getMoreConstrainedFunction(Skipped, Result))
         return nullptr;
     }
     Pair = DAP;

clang/lib/Sema/SemaTemplate.cpp

@@ -10339,24 +10339,53 @@ Sema::CheckMemberSpecialization(NamedDecl *Member, LookupResult &Previous) {
   if (Previous.empty()) {
     // Nowhere to look anyway.
   } else if (FunctionDecl *Function = dyn_cast<FunctionDecl>(Member)) {
+    SmallVector<FunctionDecl *> Candidates;
+    bool Ambiguous = false;
     for (LookupResult::iterator I = Previous.begin(), E = Previous.end();
            I != E; ++I) {
-      NamedDecl *D = (*I)->getUnderlyingDecl();
-      if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(D)) {
-        QualType Adjusted = Function->getType();
-        if (!hasExplicitCallingConv(Adjusted))
-          Adjusted = adjustCCAndNoReturn(Adjusted, Method->getType());
-        // This doesn't handle deduced return types, but both function
-        // declarations should be undeduced at this point.
-        if (Context.hasSameType(Adjusted, Method->getType())) {
-          FoundInstantiation = *I;
-          Instantiation = Method;
-          InstantiatedFrom = Method->getInstantiatedFromMemberFunction();
-          MSInfo = Method->getMemberSpecializationInfo();
-          break;
-        }
+      CXXMethodDecl *Method =
+          dyn_cast<CXXMethodDecl>((*I)->getUnderlyingDecl());
+      if (!Method)
+        continue;
+      QualType Adjusted = Function->getType();
+      if (!hasExplicitCallingConv(Adjusted))
+        Adjusted = adjustCCAndNoReturn(Adjusted, Method->getType());
+      // This doesn't handle deduced return types, but both function
+      // declarations should be undeduced at this point.
+      if (!Context.hasSameType(Adjusted, Method->getType()))
+        continue;
+      if (ConstraintSatisfaction Satisfaction;
+          Method->getTrailingRequiresClause() &&
+          (CheckFunctionConstraints(Method, Satisfaction,
+                                    /*UsageLoc=*/Member->getLocation(),
+                                    /*ForOverloadResolution=*/true) ||
+           !Satisfaction.IsSatisfied))
+        continue;
+      Candidates.push_back(Method);
+      FunctionDecl *MoreConstrained =
+          Instantiation ? getMoreConstrainedFunction(
+                              Method, cast<FunctionDecl>(Instantiation))
+                        : Method;
+      if (!MoreConstrained) {
+        Ambiguous = true;
+        continue;
+      }
+      if (MoreConstrained == Method) {
+        Ambiguous = false;
+        FoundInstantiation = *I;
+        Instantiation = Method;
+        InstantiatedFrom = Method->getInstantiatedFromMemberFunction();
+        MSInfo = Method->getMemberSpecializationInfo();
       }
     }
+    if (Ambiguous) {
+      Diag(Member->getLocation(), diag::err_function_member_spec_ambiguous)
+          << Member << (InstantiatedFrom ? InstantiatedFrom : Instantiation);
+      for (FunctionDecl *Candidate : Candidates)
+        Diag(Candidate->getLocation(), diag::note_function_member_spec_matched)
+            << Candidate;
+      return true;
+    }
   } else if (isa<VarDecl>(Member)) {
     VarDecl *PrevVar;
     if (Previous.isSingleResult() &&

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -5852,6 +5852,38 @@ UnresolvedSetIterator Sema::getMostSpecialized(
   return SpecEnd;
 }
 
+/// Returns the more constrained function according to the rules of
+/// partial ordering by constraints (C++ [temp.constr.order]).
+///
+/// \param FD1 the first function
+///
+/// \param FD2 the second function
+///
+/// \returns the more constrained function. If neither function is
+/// more constrained, returns NULL.
+FunctionDecl *Sema::getMoreConstrainedFunction(FunctionDecl *FD1,
+                                               FunctionDecl *FD2) {
+  assert(!FD1->getDescribedTemplate() && !FD2->getDescribedTemplate() &&
+         "not for function templates");
+  FunctionDecl *F1 = FD1;
+  if (FunctionDecl *MF = FD1->getInstantiatedFromMemberFunction())
+    F1 = MF;
+  FunctionDecl *F2 = FD2;
+  if (FunctionDecl *MF = FD2->getInstantiatedFromMemberFunction())
+    F2 = MF;
+  llvm::SmallVector<const Expr *, 1> AC1, AC2;
+  F1->getAssociatedConstraints(AC1);
+  F2->getAssociatedConstraints(AC2);
+  bool AtLeastAsConstrained1, AtLeastAsConstrained2;
+  if (IsAtLeastAsConstrained(F1, AC1, F2, AC2, AtLeastAsConstrained1))
+    return nullptr;
+  if (IsAtLeastAsConstrained(F2, AC2, F1, AC1, AtLeastAsConstrained2))
+    return nullptr;
+  if (AtLeastAsConstrained1 == AtLeastAsConstrained2)
+    return nullptr;
+  return AtLeastAsConstrained1 ? FD1 : FD2;
+}
+
 /// Determine whether one partial specialization, P1, is at least as
 /// specialized than another, P2.
 ///

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -275,6 +275,13 @@ Response HandleFunction(Sema &SemaRef, const FunctionDecl *Function,
                                      TemplateArgs->asArray(),
                                      /*Final=*/false);
 
+    if (RelativeToPrimary &&
+        (Function->getTemplateSpecializationKind() ==
+             TSK_ExplicitSpecialization ||
+         (Function->getFriendObjectKind() &&
+          !Function->getPrimaryTemplate()->getFriendObjectKind())))
+      return Response::UseNextDecl(Function);
+
     // If this function was instantiated from a specialized member that is
     // a function template, we're done.
     assert(Function->getPrimaryTemplate() && "No function template?");

clang/test/CXX/temp/temp.spec/temp.expl.spec/p14-23.cpp

@@ -0,0 +1,60 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+
+template<int I>
+concept C = I >= 4;
+
+template<int I>
+concept D = I < 8;
+
+template<int I>
+struct A {
+  constexpr static int f() { return 0; }
+  constexpr static int f() requires C<I> && D<I> { return 1; }
+  constexpr static int f() requires C<I> { return 2; }
+
+  constexpr static int g() requires C<I> { return 0; } // #candidate-0
+  constexpr static int g() requires D<I> { return 1; } // #candidate-1
+
+  constexpr static int h() requires C<I> { return 0; } // expected-note {{member declaration nearly matches}}
+};
+
+template<>
+constexpr int A<2>::f() { return 3; }
+
+template<>
+constexpr int A<4>::f() { return 4; }
+
+template<>
+constexpr int A<8>::f() { return 5; }
+
+static_assert(A<3>::f() == 0);
+static_assert(A<5>::f() == 1);
+static_assert(A<9>::f() == 2);
+static_assert(A<2>::f() == 3);
+static_assert(A<4>::f() == 4);
+static_assert(A<8>::f() == 5);
+
+template<>
+constexpr int A<0>::g() { return 2; }
+
+template<>
+constexpr int A<8>::g() { return 3; }
+
+template<>
+constexpr int A<6>::g() { return 4; } // expected-error {{ambiguous member function specialization 'A<6>::g' of 'A::g'}}
+                                      // expected-note@#candidate-0 {{member function specialization matches 'g'}}
+                                      // expected-note@#candidate-1 {{member function specialization matches 'g'}}
+
+static_assert(A<9>::g() == 0);
+static_assert(A<1>::g() == 1);
+static_assert(A<0>::g() == 2);
+static_assert(A<8>::g() == 3);
+
+template<>
+constexpr int A<4>::h() { return 1; }
+
+template<>
+constexpr int A<0>::h() { return 2; } // expected-error {{out-of-line definition of 'h' does not match any declaration in 'A<0>'}}
+
+static_assert(A<5>::h() == 0);
+static_assert(A<4>::h() == 1);