Implement P2167R3 Improving boolean-testable Usage (for comparison fallback CPOs)

[frederick-vs-ja]

Fixes #3211. I believe that these changes should be applied to C++20 mode, as the paper is very near to an LWG DR.

No change is needed for the rest of P2167R3, because it's "something that increases the restrictions placed on users, but implementers aren't expected to enforce those restrictions".

[strega-nil-ms]

Marking decision needed as this changes C++20 code; I personally believe that it should be treated as a DR tho.

[StephanTLavavej]

I'm in favor of implementing this basically-a-bugfix paper in C++20 mode. This is consistent with our usual policy: major features (especially those introducing any new names) are always guarded, but if it's a similar size to an LWG issue resolution, avoids disrupting all but the most pathological user code, and affects existing product code that would be a pain to maintain two variants of, then implementing it downlevel is reasonable. (The enable_shared_from_this overhaul is the prime example, and this feels very similar.)

[strega-nil-ms]

Alright, I'm good with that then.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for investigating and implementing the necessary changes here! ✅ 🕵️ 🚀