[libc++] [P1614] Implement std::compare_three_way.

[• Quuxplusone]

Summary

std::strong_order() etc. depend on this. Let's get this shipped!

[rarutyun]

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

10

Some steps in CI failed. I believe adding // UNSUPPORTED: libcpp-no-concepts should help

34

Why do we check std::compare_three_way_result_t in this test?

47

I would suggest to also test for std::partial_ordering::unordered and also add some tests for weak_ordering

50

For heterogeneous case does it make sense to put int and double in reverse order?

[rarutyun]

Should we add test to check availability from <functional> header or it's already covered by this one libcxx/test/std/utilities/function.objects/comparisons/transparent.pass.cpp?

[Mordante]

libcxx/include/__compare/compare_three_way.h

30

Nit: s/_LIBCPP_INLINE_VISIBILITY/_LIBCPP_HIDE_FROM_ABI/

32

The Standard doesn't specify the noexcept, is this added intentionally?

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

47

+1

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

60

Nit: the empty string isn't required in C++20.

[• Quuxplusone]

Address review comments; add test for <functional>.

libcxx/include/__compare/compare_three_way.h

32

Yes, for consistency with std::less<void> etc.
https://eel.is/c++draft/function.objects#comparisons doesn't put the conditional-noexcept on those either, but libc++ does. (That is, libc++ puts conditional-noexcept only on the transparent less<void>; not on less<T>.)

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

34

Just sanity-checking. NotThreeWayComparable is an example of a type where it is legal to ask for its compare_three_way_result_t, but in fact if you try to call compare_three_way on it, it doesn't compile. (But the actual relevant test is down on line 64; I should move them closer together, but I have no good idea how to.)

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

60

Yeah, but consistency with the earlier lines (including the wonky spacing).

[Mordante]

LGTM!

[ldionne]

libcxx/include/functional

139

[ldionne]

LGTM but I have a nitpick fix (see comment) and please poke CI!

[• Quuxplusone]

Update comment and poke buildkite.

[• Quuxplusone]

Remove // -*- C++ -*- comment from generated file.

[• Quuxplusone]

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

61

Bother. @ldionne, is there anything clever we could do here, to avoid splitting this one line out into a separate test marked UNSUPPORTED: libcpp-no-concepts? I'll split it out if I need to, but I'm hoping that there's a clever approach I'm forgetting about.
std::compare_three_way does not exist unless !defined(_LIBCPP_HAS_NO_CONCEPTS).

[• Quuxplusone]

Split out the is_transparent test into its own file; mark it UNSUPPORTED: libcpp-no-concepts.
I'm expecting this to pass buildkite, and then land it tonight or tomorrow.