Implement P2655R3 common_reference_t Of reference_wrapper Should Be A Reference Type

[frederick-vs-ja]

Fixes #3444.

I think the changes should be applied to C++20 mode as this documentation said P2655 is possibly a DR.

Also

• moves convertible_to from <concepts> to <type_traits> for convenience,
• drops section numbers in <type_traits> (towards STL: Update comments citing old Working Papers #182), and
• updates references to WD to WG21-N4928 in <type_traits> and VSO_0000000_type_traits/test.cpp.

[strega-nil-ms]

Minor questions, otherwise looks good!

[StephanTLavavej]

Thanks, this looks solid, and I agree that implementing it in C++20 mode is reasonable! 😻 (This aligns with our usual rationale - if a paper deeply modifies an existing feature in a way that doesn't introduce new names, isn't likely to be source-breaking, is generally experienced as a strict improvement, and would be obnoxious to conditionally maintain, then implementing it retroactively is desirable. This is very much like the enable_shared_from_this overhaul.)

I've pushed some nitpicky cosmetic changes, nothing functional. FYI @strega-nil-ms as you already approved.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for enhancing some of the STL's most intricate template machinery! ⚙️ ✨ 🚀