[libcxx-commits] [libcxx] [libc++] Fix instantiation of incomplete type when evaluating tuple comparisons (PR #204679)

[A. Jiang via libcxx-commits]

https://github.com/frederick-vs-ja approved this pull request.

LGTM!

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### Off-topic

BTW, I'm a bit afraid that the current constraints don't properly handle move-only `subrange` types (whose iterator types are move-only). But for these types, the `std::get` calls (unfortunately necessary for comparison due to [[tuple.rel]](https://eel.is/c++draft/tuple.rel)) are already weird - as they consume the iterators.

Perhaps we should make comparison between `tuple` and `subrange` never copy or move iterators (and sentinels). Such fix seemingly requires an LWG issue. Anyway, it shouldn't be related to this PR.

https://github.com/llvm/llvm-project/pull/204679