Ignore calling conventions when checking function template specializations

[rnk]

Summary

Calling conventions are inherited during decl merging. Before this
change, deduction would fail due to a type mismatch between the template
and the specialization. This change adjusts the CCs to match before
deduction, and lets the decl merging logic diagnose mismatch or inherit
the CC from the template.

This allows specializations of static member function templates in the
Microsoft C++ ABI.

[rnk]

Ping.

[rsmith]

LGTM

We should probably do the same thing for 'noreturn' :-)

[rnk]

Closed by commit rL190377 (authored by @rnk).

cfe/trunk/lib/Sema/SemaTemplate.cpp

if (FunctionTemplateDecl *FunTmpl = dyn_cast<FunctionTemplateDecl>(Ovl)) {
         continue;
 
       // When matching a constexpr member function template specialization
       // against the primary template, we don't yet know whether the
       // specialization has an implicit 'const' (because we don't know whether
       // it will be a static member function until we know which template it
       // specializes), so adjust it now assuming it specializes this template.
       QualType FT = FD->getType();
+      const FunctionProtoType *FPT = FT->castAs<FunctionProtoType>();
+      FunctionDecl *TmplFD = FunTmpl->getTemplatedDecl();
       if (FD->isConstexpr()) {
-        CXXMethodDecl *OldMD =
-          dyn_cast<CXXMethodDecl>(FunTmpl->getTemplatedDecl());
+        CXXMethodDecl *OldMD = dyn_cast<CXXMethodDecl>(TmplFD);
         if (OldMD && OldMD->isConst()) {
-          const FunctionProtoType *FPT = FT->castAs<FunctionProtoType>();
           FunctionProtoType::ExtProtoInfo EPI = FPT->getExtProtoInfo();
           EPI.TypeQuals |= Qualifiers::Const;
           FT = Context.getFunctionType(FPT->getResultType(), FPT->getArgTypes(),
                                        EPI);
         }
       }
 
+      // Ignore differences in calling convention until decl merging.
+      const FunctionProtoType *TmplFT =
+          TmplFD->getType()->castAs<FunctionProtoType>();
+      if (FPT->getCallConv() != TmplFT->getCallConv()) {
+        FunctionProtoType::ExtProtoInfo EPI = FPT->getExtProtoInfo();
+        EPI.ExtInfo = EPI.ExtInfo.withCallingConv(TmplFT->getCallConv());
+        FT = Context.getFunctionType(FPT->getResultType(), FPT->getArgTypes(),
+                                     EPI);
+      }
+
       // C++ [temp.expl.spec]p11:
       //   A trailing template-argument can be left unspecified in the
       //   template-id naming an explicit function template specialization
       //   provided it can be deduced from the function argument type.
       // Perform template argument deduction to determine whether we may be
       // specializing this template.
       // FIXME: It is somewhat wasteful to build
       TemplateDeductionInfo Info(FailedCandidates.getLocation());

cfe/trunk/test/SemaCXX/decl-microsoft-call-conv.cpp

 // Test functions with multiple attributes.
 void __attribute__((noreturn)) __stdcall __attribute__((regparm(1))) multi_attribute(int x);
 void multi_attribute(int x) { __builtin_unreachable(); }
 
 
 // expected-error@+2 {{stdcall and cdecl attributes are not compatible}}
 // expected-error@+1 {{fastcall and cdecl attributes are not compatible}}
 void __cdecl __cdecl __stdcall __cdecl __fastcall multi_cc(int x);
+
+template <typename T> void __stdcall StdcallTemplate(T) {}
+template <> void StdcallTemplate<int>(int) {}
+template <> void __stdcall StdcallTemplate<short>(short) {}
+
+// FIXME: Note the template, not the implicit instantiation.
+// expected-error@+2 {{function declared 'cdecl' here was previously declared 'stdcall}}
+// expected-note@+1 {{previous declaration is here}}
+template <> void __cdecl StdcallTemplate<long>(long) {}
+
+struct ExactlyInt {
+  template <typename T> static int cast_to_int(T) {
+    return T::this_is_not_an_int();
+  }
+};
+template <> inline int ExactlyInt::cast_to_int<int>(int x) { return x; }