[libcxx-commits] [libcxx] [libc++] Remove _LIBCPP_USING_IF_EXISTS from fundamental aliases (PR #107220)

[Louis Dionne via libcxx-commits]

https://github.com/ldionne requested changes to this pull request.

This makes sense to me for `ptrdiff_t`, `size_t` and `nullptr_t`, but not for the optional types like `int8_t`. Also this should be rebased on top of the new `__cstddef` headers.

https://github.com/llvm/llvm-project/pull/107220