[libcxx] adds std::compare_three_way

[cjdb]

Summary

[libcxx] adds std::compare_three_way

• Adds std::compare_three_way without concept-checking
• Adds test for std::compare_three_way

[miscco]

libcxx/include/functional

515

Usually this is spellec as #ifndef _LIBCPP_HAS_NO_SPACESHIP_OPERATOR

833

I had to read it twice o could we improve that comment to:
// TODO(cjdb): Needs concept three_way_comparable_with

[cjdb]

libcxx/include/functional

515

Ack, I am following convention I've seen elsewhere. lmk if this is a hard sell and I can change it.

[cjdb]

Ping.

[curdeius]

My 2 cents.

libcxx/include/functional

151

Shouldn't you keep using is_transparent = unspecified; in synopsis?

516

Shouldn't we avoid conditional includes within libc++?

libcxx/test/std/utilities/function.objects/comparisons/compare_three_way.pass.cpp

13

Nit: less? why not compare_three_way?

64

Sorry for being repetitive in my reviews :). Could you make the run-time and compile-time tests equivalent?
I imagine that this might need to get rid of reinterpret_cast from do_pointer_comparison_test when making it constexpr.

[curdeius]

libcxx/include/functional

834

I think you can use noexcept instead of _NOEXCEPT_ as it's always available in C++20.