[Bug c++/103426] New: Acceptance of invalid template specialization in a namespace not enclosing the specialized template

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103426

            Bug ID: 103426
           Summary: Acceptance of invalid template specialization in a
                    namespace not enclosing the specialized template
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

The code as follows is invalid:
```
template<typename T>
struct S {
    static int x;
};

namespace {
    template<> int S<int>::x = 0;
}
```
because an explicit specialization shall be declared in a namespace enclosing
the specialized template. Clang rejects it, but not GCC. Demo:
https://gcc.godbolt.org/z/74oxofsaf

Related discussion: https://stackoverflow.com/q/30400814/7325599