[Bug libstdc++/122738] Accepts-invalid "struct _Atomic(int)" because _Atomic is not defined as an alias template

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122738

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The suggestion from LWG was:

template <typename T>
  using std-atomic = see below; // exposition only
#define _Atomic(T) std-atomic<T>;

But the 'see below' didn't make it into the approved paper.