I've found a few places where a null memory_resource* is passed to a resource or a polymorphic_allocator, which is undefined:

[mem.poly.allocator.ctor]:

polymorphic_allocator(memory_resource* r);

2. Preconditions: r is non-null.

If this is an intentional libc++ extension then should the tests be guarded with _LIBCPP_VERSION, or moved to the libcxx/test/libcxx directory for non-standard features?