[libc++] Accepts invalid `struct _Atomic(T)` because `_Atomic` expands to a class template

[eisenwave]

https://godbolt.org/z/97x9Ms8eK

#include <stdatomic.h>
struct _Atomic(int) x;

This code should raise an error (or warning), but is falsely accepted by Clang.

https://eel.is/c++draft/stdatomic.h.syn specifies _Atomic as expanding to an exposition-only std-atomic alias template. This forbids the use of a struct keyword.

The bug occurs because libc++ defines _Atomic like:

#define _Atomic(_Tp) ::std::atomic<_Tp>

llvm-project/libcxx/include/stdatomic.h

Lines 134 to 138 in 7c34848

	# ifdef _Atomic
	# undef _Atomic
	# endif
	# define _Atomic(_Tp) ::std::atomic<_Tp>

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