new_delete_resource().allocate(n, a) has basically two permissible results:
- Return an appropriately sized and aligned block.
- Throw bad_alloc.
Before this patch, libc++'s new_delete_resource would do a third and impermissible thing, which was to return an appropriately sized but inappropriately under-aligned block. This is now fixed.
(This came up while I was stress-testing unsynchronized_pool_resource on my MacBook. If we can't trust the default resource to return appropriately aligned blocks, pretty much everything breaks. For similar reasons, I would strongly support just patching __libcpp_allocate directly, but I don't care to die on that hill, so I made this patch as a <memory_resource>-specific workaround.)