Proposed resolution:

This wording is relative to N4296.

1. Change [allocator.requirements], Table 28 — "Allocator requirements" as presented:

   Table 28 — Allocator requirements

   Expression	Return type	Assertion/note pre-/post-condition	Default
   …
   X::is_always_equal	Identical to or derived from true_type or false_type	[…]	is_empty<X>::type
   …