[Bug c++/86254] New: g++ rejects legal code?

[zhonghao at pku dot org.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86254

            Bug ID: 86254
           Summary: g++ rejects legal code?
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhonghao at pku dot org.cn
  Target Milestone: ---

The code is as follow:

namespace N {
 extern "C" {
 extern const int foobar;
 const int foobar = 1;
 struct S { static const int foobar; };
 const int S::foobar = 2;
 }
}
int main () { return !(N::foobar + 1 == N::S::foobar); }

clang++ accepts the code, but g++ produces error messages:

conflicting declaration of ‘const int N::S::foobar’ with ‘C’ linkage
      const int S::foobar = 2;
previous declaration with ‘C++’ linkage
      struct S { static const int foobar; };

Indeed, the code comes from a bug report of gcc
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33786). A previous version of g++
produces a different error message, but was fixed in the latest version. 

I reported this as a bug in clang: https://bugs.llvm.org/show_bug.cgi?id=37844

However, Richard Smith told me that gcc is wrong. He cited a sentence "A C
language linkage is ignored in determining the language linkage of the names of
class members" to support his statement. This sounds like a specification. So,
is this really a bug in gcc?