P2538R1 ADL-Proof `projected`

[StephanTLavavej]

WG21-P2538R1 ADL-Proof projected

This paper deliberately doesn't provide a feature-test macro.

Note: We're focused on implementing the remaining library-only features in C++23. Until that's done, we will NOT be accepting PRs for C++26 features.

[frederick-vs-ja]

It seems to me that we should implement this in C++20 mode, like WG21-P2602R2 (#3209) and WG21-P2609R3 (#3441).

[StephanTLavavej]

I would be fine with that if the other maintainers are, in which case this would be a special exception to our "not yet" policy.