LWG3315: Correct Allocator Default Behavior

[cjdb]

Link: https://wg21.link/lwg3315

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Christopher Di Bella (cjdb)

Details

**Link:** https://wg21.link/lwg3315

[ldionne]

Per #105593 (comment):

IIUC LWG3315 can only be considered as a patch for P0784R7/C++20, not to earlier standard revisions, and the operations in "Default" are specified for allocator_traits, which has been implemented in LLVM 12.0.

So IMO we've already implemented this issue by making sure that we call construct_at and destroy_at inside allocator_traits. This is a subtle one.