Proposed resolution:

Wording relative to N4700.

1. Add a new paragraph in [mem.poly.allocator.class] after para 1:

   1 A specialization of class template pmr::polymorphic_allocator conforms to the Allocator requirements [...]

   -?- All specializations of class template pmr::polymorphic_allocator satisfy the allocator completeness requirements ([allocator.requirements.completeness]).