[libcxx-commits] [libcxx] [libc++] __key_equiv is sometimes unused, sometimes 2x expensive (PR #175087)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 requested changes to this pull request.

I don't think this is a good idea, at least as-is. `ranges::unique` requires the predicate to be an equivalence relation, which the new definition of `__key_equiv` isn't, which means that this patch introduces UB.

https://github.com/llvm/llvm-project/pull/175087