PR #154512 (relanded by #155565) removed `__can_extract_map_key`, which
had a blanket opt-out for `std::*set`s. The new logic does not have that
opt-out, leading to `std::set`s being incorrectly constructed.
The new regression tests demonstrate this, but essentially the idea is:
1. Have `std::set<T> foo;`
2. Call `foo.emplace(some_t,
arg_that_influences_comparisons_or_hashes);`
3. The emplace will internally search using `some_t` as the key, *not*
`T(some_t, arg_that_influences_comparisons_or_hashes);`
This opts out `std::*set` from this optimization to match previous
behavior.
Tests and fix were produced by an LLM. I reviewed them and they seem
reasonable to me, though I don't have a strong background in libc++
testing conventions.
(Credit to Deva S, our partner who tracked this down. Unfortunately I
don't know his GH handle)
Fixes#220451
(cherry picked from commit 10d4bdf93a82fb290b8c55da6ed19ff54da67296)
George Burgess IV
2eabe80bc7