[Bug libstdc++/119620] flat_set::emplace is constrained, and always constructs element on the stack

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119620

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
... and that's what "Cpp17EmplaceConstructible into X" is about.

But we can't constrain based on that, and can't spell out exactly what kind of
initialization is done, because we don't know. Only the allocator knows, and
it's not talking (because Alloc::construct is not necessarily constrained to
let us know if Args... can be used).