[FOLD] improve ambiguity diagnostic

Author: sdkrystian

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5438,7 +5438,7 @@ 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 of %0">;
+    "ambiguous member function specialization %q0 of %q1">;
 def note_function_member_spec_matched : Note<
     "member function specialization matches %0">;
 

clang/lib/Sema/SemaTemplate.cpp

@@ -10380,7 +10380,8 @@ Sema::CheckMemberSpecialization(NamedDecl *Member, LookupResult &Previous) {
     }
     if (Ambiguous) {
       Diag(Member->getLocation(), diag::err_function_member_spec_ambiguous)
-          << Member;
+          << Member
+          << (InstantiatedFrom ? InstantiatedFrom : Instantiation);
       for (FunctionDecl *Candidate : Candidates)
         Diag(Candidate->getLocation(), diag::note_function_member_spec_matched)
             << Candidate;

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

@@ -41,7 +41,7 @@ template<>
 constexpr int A<8>::g() { return 3; }
 
 template<>
-constexpr int A<6>::g() { return 4; } // expected-error {{ambiguous member function specialization of 'g'}}
+constexpr int A<6>::g() { return 4; } // expected-error {{ambiguous member function specialization 'A<6>::g' of 'A::g'}}
 
 static_assert(A<9>::g() == 0);
 static_assert(A<1>::g() == 1);