[Clang][Sema] Fix explicit specializations of member function templates with a deduced return type (#86817)

Clang erroneously rejects the following:
```
template<typename T>
struct A
{
    template<typename U>
    auto f();
};

template<>
template<typename U>
auto A<int>::f(); // error: conflicting types for 'f'
```
This happens because the explicit specialization of `f` has its return
type replaced with a dependent `AutoType` in `ActOnFunctionDeclarator`,
but no such replacement occurs for the implicitly instantiated function
template `A<int>::f`. Since the return types don't match, the explicit
specialization is diagnosed as an invalid redeclaration.

This patch moves the replacement of the return type to
`CheckFunctionDeclaration` so it also happens during instantiation.
`setObjectOfFriendDecl` will have been called by then, so the `isFriend
&& CurContext->isDependentContext()` condition is made redundant &
removed (as it already happens in `DeclContext::isDependentContext`).
`Sema::IsOverload` only checks the _declared_ return type (which isn't
changed by the adjustment), so adjusting the return type afterwards
should be safe.

1 files changed, 29 insertions, 17 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 6ff85c0..5c11528 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -10124,23 +10124,6 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
         Diag(D.getDeclSpec().getVirtualSpecLoc(), diag::err_auto_fn_virtual);
     }
 
-    if (getLangOpts().CPlusPlus14 &&
-        (NewFD->isDependentContext() ||
-         (isFriend && CurContext->isDependentContext())) &&
-        NewFD->getReturnType()->isUndeducedType()) {
-      // If the function template is referenced directly (for instance, as a
-      // member of the current instantiation), pretend it has a dependent type.
-      // This is not really justified by the standard, but is the only sane
-      // thing to do.
-      // FIXME: For a friend function, we have not marked the function as being
-      // a friend yet, so 'isDependentContext' on the FD doesn't work.
-      const FunctionProtoType *FPT =
-          NewFD->getType()->castAs<FunctionProtoType>();
-      QualType Result = SubstAutoTypeDependent(FPT->getReturnType());
-      NewFD->setType(Context.getFunctionType(Result, FPT->getParamTypes(),
-                                             FPT->getExtProtoInfo()));
-    }
-
     // C++ [dcl.fct.spec]p3:
     //  The inline specifier shall not appear on a block scope function
     //  declaration.
@@ -12112,6 +12095,35 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
 
   CheckConstPureAttributesUsage(*this, NewFD);
 
+  // C++ [dcl.spec.auto.general]p12:
+  //   Return type deduction for a templated function with a placeholder in its
+  //   declared type occurs when the definition is instantiated even if the
+  //   function body contains a return statement with a non-type-dependent
+  //   operand.
+  //
+  // C++ [temp.dep.expr]p3:
+  //   An id-expression is type-dependent if it is a template-id that is not a
+  //   concept-id and is dependent; or if its terminal name is:
+  //   - [...]
+  //   - associated by name lookup with one or more declarations of member
+  //     functions of a class that is the current instantiation declared with a
+  //     return type that contains a placeholder type,
+  //   - [...]
+  //
+  // If this is a templated function with a placeholder in its return type,
+  // make the placeholder type dependent since it won't be deduced until the
+  // definition is instantiated. We do this here because it needs to happen
+  // for implicitly instantiated member functions/member function templates.
+  if (getLangOpts().CPlusPlus14 &&
+      (NewFD->isDependentContext() &&
+       NewFD->getReturnType()->isUndeducedType())) {
+    const FunctionProtoType *FPT =
+        NewFD->getType()->castAs<FunctionProtoType>();
+    QualType NewReturnType = SubstAutoTypeDependent(FPT->getReturnType());
+    NewFD->setType(Context.getFunctionType(NewReturnType, FPT->getParamTypes(),
+                                           FPT->getExtProtoInfo()));
+  }
+
   // C++11 [dcl.constexpr]p8:
   //   A constexpr specifier for a non-static member function that is not
   //   a constructor declares that member function to be const.