[libcxx-commits] [libcxx] [libc++] Don't implement <stdatomic.h> before C++23 (PR #123130)

[Louis Dionne via libcxx-commits]

ldionne wrote:

Another example of valid code that works without the extension and breaks with the extension:

```c++
// REQUIRES: c++03 || c++11 || c++14 || c++17 || c++20

struct Foo {
  _Atomic(int) f() const;
};

#include <stdatomic.h>

_Atomic(int) Foo::f() const; // This declaration would differ from the previous one in C++23
```

https://github.com/llvm/llvm-project/pull/123130