[libcxx-commits] [libcxx] [libcxxabi] [libc++][RFC] Always define internal feature test macros (PR #89178)

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EricWF wrote:

> While the practice is long-standing, the names of the individual macros changed before ([cf65d27](https://github.com/llvm/llvm-project/commit/cf65d275ac048dfb781c7302c6c82c3500184eb1), [b22aa3d](https://github.com/llvm/llvm-project/commit/b22aa3d74f276afbfbd10953de68e3b4f8cef907), [5c40c99](https://github.com/llvm/llvm-project/commit/5c40c994c329aa7e521c53cbc893426c90203941), [66a562d](https://github.com/llvm/llvm-project/commit/66a562d22e708ba40b8443b58e504ac3f983ba59)), without any major fallout I'm aware of. So I don't think a clang-tidy check pulls its weight here. A general check for internal macros might be nice, but is definitely out of scope for this change.

I don't believe any of those changes changed the way the macro works. They simply re-named things. So I wouldn't expect them to have "fallout" in the way I'm suggesting.

Also, if the purpose of this change is entirely for bug prevention & macro misuse, then a tool to assist with that is well in scope. 


https://github.com/llvm/llvm-project/pull/89178