[Clang][Sema] Improve support for explicit specializations of constra…

…ined member functions & member function templates (llvm#88963)

Consider the following snippet from the discussion of CWG2847 on the core reflector:
```
template<typename T>
concept C = sizeof(T) <= sizeof(long);

template<typename T>
struct A 
{
    template<typename U>
    void f(U) requires C<U>; // #1, declares a function template 

    void g() requires C<T>; // rust-lang#2, declares a function

    template<>
    void f(char);  // rust-lang#3, an explicit specialization of a function template that declares a function
};

template<>
template<typename U>
void A<short>::f(U) requires C<U>; // rust-lang#4, an explicit specialization of a function template that declares a function template

template<>
template<>
void A<int>::f(int); // rust-lang#5, an explicit specialization of a function template that declares a function

template<>
void A<long>::g(); // rust-lang#6, an explicit specialization of a function that declares a function
```

A number of problems exist:
- Clang rejects `rust-lang#4` because the trailing _requires-clause_ has `U`
substituted with the wrong template parameter depth when
`Sema::AreConstraintExpressionsEqual` is called to determine whether it
matches the trailing _requires-clause_ of the implicitly instantiated
function template.
- Clang rejects `rust-lang#5` because the function template specialization
instantiated from `A<int>::f` has a trailing _requires-clause_, but `rust-lang#5`
does not (nor can it have one as it isn't a templated function).
- Clang rejects `rust-lang#6` for the same reasons it rejects `rust-lang#5`.

This patch resolves these issues by making the following changes:
- To fix `rust-lang#4`, `Sema::AreConstraintExpressionsEqual` is passed
`FunctionTemplateDecl`s when comparing the trailing _requires-clauses_
of `rust-lang#4` and the function template instantiated from `#1`.
- To fix `rust-lang#5` and `rust-lang#6`, the trailing _requires-clauses_ are not compared
for explicit specializations that declare functions.

In addition to these changes, `CheckMemberSpecialization` now considers
constraint satisfaction/constraint partial ordering when determining
which member function is specialized by an explicit specialization of a
member function for an implicit instantiation of a class template (we
previously would select the first function that has the same type as the
explicit specialization). With constraints taken under consideration, we
match EDG's behavior for these declarations.

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);