P2875R4 Undeprecate `polymorphic_allocator::destroy`

[StephanTLavavej]

WG21-P2875R4 Undeprecate polymorphic_allocator::destroy

No feature-test macro is mentioned.

This wasn't formally accepted as a Defect Report, but I believe that we should implement it retroactively.

[frederick-vs-ja]

In addition to undeprecation, P2875R4 also changes "As if by" to "Equivalent to".

So, do we also need to constrain polymorphic_allocator::destroy? (We may already need to do this if LWG-3401 is accepted.)

No, SFINAE constraints are not introduced because currently only Constraints specified in the library wording is propagated.