Implement P2445R1 forward_like()

[A. Jiang (frederick-vs-ja)]

Fixes#2931.

Intentionally don't call std::move, std::forward, or std::as_const. Also move _Can_reference to <utility> for error message.

[Casey Carter (CaseyCarter)]

LGTM!

[nicole mazzuca (strega-nil-ms)]

I don't really have a "problem" with how it's written right now, but I would prefer it if this was written closer to the wording of the paper; something like:

using _UnrefT = remove_reference_t<_Ty>;
using _UnrefU = remove_reference_t<_Uty>;
ifconstexpr (is_const_v<_UnrefT>) {
// _Copy_const = const _UnrefUifconstexpr (is_rvalue_reference_v<_Ty&&>) {
returnstatic_cast<const _UnrefU&&>(_Ux);
} else {
returnstatic_cast<const _UnrefU&>(_Ux);
}
} else {
// _Copy_const = _UnrefUifconstexpr (is_rvalue_reference_v<_Ty&&>) {
returnstatic_cast<_UnrefU&&>(_Ux);
} else {
returnstatic_cast<_UnrefU&>(_Ux);
}
}

[Stephan T. Lavavej (StephanTLavavej)]

A. Jiang (@frederick-vs-ja)Casey Carter (@CaseyCarter)nicole mazzuca (@strega-nil-ms) Thanks, this looks good! I added a preprocessor comment and slightly expanded the test coverage (mostly out of principle, not any real concern about library or compiler bugs).

[Stephan T. Lavavej (StephanTLavavej)]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[Stephan T. Lavavej (StephanTLavavej)]

Thanks for implementing this feature - I like to see the STL moving forward! 😹 😻 🚀