this->get_allocator()== rhs.get_allocator() is true when
allocator_traits<Allocator>::propagate_on_container_move_assignment::value
is true; otherwise, the allocator is unchanged.
Preconditions: Either
allocator_traits<Allocator>::propagate_on_container_swap::value
is true
or
this->get_allocator()== other.get_allocator()
is true.