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

[Ryan Prichard via libcxx-commits]

rprichard wrote:

Yeah, removing the extension from upstream seems OK to me. I suspect the Android toolchain situation around stdatomic.h will continue to be complicated, but maybe we can upgrade enough stuff to C++23 and drop Android's historic extension?

> 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
> ```

FWIW, I think this code is accepted by Clang prior to the stdatomic.h extension PR, but not by other compilers. i.e. It's a Clang extension to allow C11-style atomics in a C++ language dialect (as long as `<atomic>` isn't included!).

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