Downgrade the error about nested name specifiers when building with Microsoft extensions

[• ehsan]

Summary

This fixes http://llvm.org/PR20145.

[• ehsan]

Renamed the variable to ext_...

[majnemer]

Usually we only downgrade this kind of error if it was blocking a system header, code that was automatically generated by some sort of IDL, or if it is impractical to change the code in some other way. Is that the case here?

It's preferable, in my eyes, to keep it an error so that programmers make their code conforming instead of just ignoring another warning fly by on their terminal.

I don't think clang-cl should mirror every bug in cl, just the important ones.

[rnk]

I'd echo what David said, but otherwise, this seems fine.

lib/Sema/SemaDecl.cpp

3290

We use LangOpts.MSVCCompat to control these language bending extensions.

[• ehsan]

We hit this error in our code and I have since fixed the code on our side.
I'm not aware of any system headers or generate code that hits this.

Based on that, should we can this patch?

Ehsan
http://ehsanakhgari.org/

[• ehsan]

We decided to can this.

include/clang/Basic/DiagnosticSemaKinds.td

def err_typecheck_sclass_fscope : Error<
   "illegal storage class on file-scoped variable">;
 def warn_standalone_specifier : Warning<"'%0' ignored on this declaration">,
   InGroup<MissingDeclarations>;
 def ext_standalone_specifier : ExtWarn<"'%0' is not permitted on a declaration "
   "of a type">, InGroup<MissingDeclarations>;
 def err_standalone_class_nested_name_specifier : Error<
   "forward declaration of %select{class|struct|interface|union|enum}0 cannot "
   "have a nested name specifier">;
+def ext_standalone_class_nested_name_specifier : ExtWarn<
+  "forward declaration of %select{class|struct|interface|union|enum}0 with a "
+  "nested name specifier is a Microsoft extension">, InGroup<Microsoft>;
 def err_typecheck_sclass_func : Error<"illegal storage class on function">;
 def err_static_block_func : Error<
   "function declared in block scope cannot have 'static' storage class">;
 def err_typecheck_address_of : Error<"address of %select{bit-field"
   "|vector element|property expression|register variable}0 requested">;
 def ext_typecheck_addrof_void : Extension<
   "ISO C forbids taking the address of an expression of type 'void'">;
 def err_unqualified_pointer_member_function : Error<

lib/Sema/SemaDecl.cpp

Decl *Sema::ParsedFreeStandingDeclSpec(Scope *S, AccessSpecifier AS,
   bool IsExplicitSpecialization =
     !TemplateParams.empty() && TemplateParams.back()->size() == 0;
   if (Tag && SS.isNotEmpty() && !Tag->isCompleteDefinition() &&
       !IsExplicitInstantiation && !IsExplicitSpecialization) {
     // Per C++ [dcl.type.elab]p1, a class declaration cannot have a
     // nested-name-specifier unless it is an explicit instantiation
     // or an explicit specialization.
     // Per C++ [dcl.enum]p1, an opaque-enum-declaration can't either.
-    Diag(SS.getBeginLoc(), diag::err_standalone_class_nested_name_specifier)
+    Diag(SS.getBeginLoc(),
+         getLangOpts().MicrosoftExt ?
+           diag::ext_standalone_class_nested_name_specifier :
+           diag::err_standalone_class_nested_name_specifier)
       << (DS.getTypeSpecType() == DeclSpec::TST_class ? 0 :
           DS.getTypeSpecType() == DeclSpec::TST_struct ? 1 :
           DS.getTypeSpecType() == DeclSpec::TST_interface ? 2 :
           DS.getTypeSpecType() == DeclSpec::TST_union ? 3 : 4)
       << SS.getRange();
     return nullptr;
   }
 

test/SemaCXX/MicrosoftExtensions.cpp

void AfterClassBody() {
   struct D {} __declspec(deprecated);
 
   struct __declspec(align(4)) S {} __declspec(align(8)) s1;
   S s2;
   _Static_assert(__alignof(S) == 4, "");
   _Static_assert(__alignof(s1) == 8, "");
   _Static_assert(__alignof(s2) == 4, "");
 }
+
+namespace forward_decl_nested_name_NS {
+  struct X;
+  template<typename T> struct Y;
+}
+
+struct forward_decl_nested_name_NS::X; // expected-warning {{forward declaration of struct with a nested name specifier is a Microsoft extension}}
+template<typename T> struct forward_decl_nested_name_NS::Y; // expected-warning {{forward declaration of struct with a nested name specifier is a Microsoft extension}}
+
+struct forward_decl_nested_name_struct {
+  struct A;
+  union B;
+};
+
+struct forward_decl_nested_name_struct::A; // expected-warning {{forward declaration of struct with a nested name specifier is a Microsoft extension}}
+union forward_decl_nested_name_struct::B; // expected-warning {{forward declaration of union with a nested name specifier is a Microsoft extension}}