[Bug libstdc++/64913] basic_string fails to use custom allocator

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64913

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That's not a valid allocator,
allocator_traits<my_allocator<X>>::rebind_alloc<X> produces std::allocator<X>
but it should be my_allocator<X>