[libcxx-commits] [libcxx] [libc++] Drop support for the C++20 Synchronization Library before C++20 (PR #82008)

[Louis Dionne via libcxx-commits]

ldionne wrote:

> Do we really need to drop support for all versions prior to C++20? For example I've seen the `<barrier>` header used in code bases. My main concern is removing support without our typical deprecation heads-up.

I would definitely drop it in all versions before C++20. Since we're going through the trouble of this deprecation + removal, I'd remove it in all standard modes it shouldn't be in.

Since LLVM 18 has shipped, what I suggest is to mark the library as deprecated < C++20 in LLVM 19, and to remove it in LLVM 20. I'll create another PR for the deprecation.

https://github.com/llvm/llvm-project/pull/82008