[libc++] Implements concept default_initializable.

[Mordante]

Summary

Implements:

• LWG3149 DefaultConstructible should require default initialization

Implements parts of:

• P0898R3 Standard Library Concepts
• P1754 Rename concepts to standard_case for C++20, while we still can

Depends on D91986

[Mordante]

libcxx/test/std/concepts/concept.default.init/default_initializable.compile.pass.cpp

49

Remove this test. During the review of D91986 is turned out this deprecated feature is no longer valid in C++20.

[cjdb]

Will give a more thorough review next week, but a quick weekend one LGTM.

[ldionne]

This LGTM except for the nitpick.

libcxx/test/std/concepts/concept.default.init/default_initializable.verify.cpp

34

These implementation details of libc++ shouldn't be tested here, or if they are, then we should use LIBCPP_STATIC_ASSERT (which expands to nothing when testing a non-libc++ library).

[Mordante]

libcxx/test/std/concepts/concept.default.init/default_initializable.verify.cpp

34

I'll use the LIBCPP_STATIC_ASSERT since during debugging the code it was good to see whether the expected test failed.

[Mordante]

• Removed the throw()
• Target libc++ release 13 instead of 12
• Changed internal tests to use LIBCPP_STATIC_ASSERT
• rebased

[ldionne]

Ship it once CI is done! Thanks!

[Mordante]

Rebase to trigger CI.