C++23 becomes allocator<*>::allocate_at_least:
https://en.cppreference.com/w/cpp/memory/allocator/allocate_at_least
It returns sth. like a pair with T * and size-t from which a container
could see how much memory is actually to adjust its actual size-parame-
ter. That's especially good for alloctos which have size classes that
aren't linear for small sizes but a power of two.