[MSVC2012] Allow 'mutable' references (fix for http://llvm.org/PR22444)

[ABataev]

Summary

Some standard header files from MSVC2012 use 'mutable' on references, though it is directly prohibited by the standard.

[rsmith]

Please emit a warning when this extension is used. If we're only doing this
to support a system header, you could even make it a DefaultError ExtWarn.
Other than that, this looks fine.

[thakis]

Out of interest, which header file? We are able to parse all of Chromium
with MSVC2012 a while ago, and that uses lots of Windows headers.

[rnk]

In D7370#117647, @thakis wrote:

Out of interest, which header file? We are able to parse all of Chromium
with MSVC2012 a while ago, and that uses lots of Windows headers.

Chromium doesn't use C++11 library features like <future>, which tend to pull in other ConCRT headers that we aren't testing yet.

cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td

def ext_nested_name_spec_is_enum : ExtWarn<
   "use of enumeration in a nested name specifier is a C++11 extension">,
   InGroup<CXX11>;
 
 // C++ class members
 def err_storageclass_invalid_for_member : Error<
   "storage class specified for a member declaration">;
 def err_mutable_function : Error<"'mutable' cannot be applied to functions">;
 def err_mutable_reference : Error<"'mutable' cannot be applied to references">;
+def ext_mutable_reference : ExtWarn<
+  "'mutable' on a reference type is a Microsoft extension">,
+  InGroup<Microsoft>;
 def err_mutable_const : Error<"'mutable' and 'const' cannot be mixed">;
 def err_mutable_nonmember : Error<
   "'mutable' can only be applied to member variables">;
 def err_virtual_non_function : Error<
   "'virtual' can only appear on non-static member functions">;
 def err_virtual_out_of_class : Error<
   "'virtual' can only be specified inside the class definition">;
 def err_virtual_member_function_template : Error<

cfe/trunk/lib/Sema/SemaDecl.cpp

if (!BitWidth) {
       ZeroWidth = false;
     }
   }
 
   // Check that 'mutable' is consistent with the type of the declaration.
   if (!InvalidDecl && Mutable) {
     unsigned DiagID = 0;
     if (T->isReferenceType())
-      DiagID = diag::err_mutable_reference;
+      DiagID = getLangOpts().MSVCCompat ? diag::ext_mutable_reference
+                                        : diag::err_mutable_reference;
     else if (T.isConstQualified())
       DiagID = diag::err_mutable_const;
 
     if (DiagID) {
       SourceLocation ErrLoc = Loc;
       if (D && D->getDeclSpec().getStorageClassSpecLoc().isValid())
         ErrLoc = D->getDeclSpec().getStorageClassSpecLoc();
       Diag(ErrLoc, DiagID);
+      if (DiagID != diag::ext_mutable_reference) {
         Mutable = false;
         InvalidDecl = true;
       }
     }
+  }
 
   // C++11 [class.union]p8 (DR1460):
   //   At most one variant member of a union may have a
   //   brace-or-equal-initializer.
   if (InitStyle != ICIS_NoInit)
     checkDuplicateDefaultInit(*this, cast<CXXRecordDecl>(Record), Loc);
 
   FieldDecl *NewFD = FieldDecl::Create(Context, Record, TSSL, Loc, II, T, TInfo,

cfe/trunk/test/SemaCXX/ms_mutable_reference_member.cpp

+// RUN: %clang_cc1 %s -fsyntax-only -verify -fms-compatibility
+
+struct S {
+  mutable int &a; // expected-warning {{'mutable' on a reference type is a Microsoft extension}}
+  S(int &b) : a(b) {}
+};
+
+int main() {
+  int a = 0;
+  const S s(a);
+  s.a = 10;
+  return s.a + a;
+}