How the allocator is propagated on swap
Declared in <bslma_allocatortraits.h>
<bslma_allocatortraits.h>
using propagate_on_container_swap = __detected_or_t<false_type, __pocs, _Alloc>;
Alloc::propagate_on_container_swap if that type exists, otherwise false_type
Alloc::propagate_on_container_swap
false_type