Inspired by Volodymyr's work on D48753, I've taken it upon myself to refactor the static member functions std::allocator_traits<A>::__construct_forward, __construct_backward, and __construct_range_forward into non-member functions. I think this is reasonable just in terms of code-cleanliness — they don't *have* to be member functions for any reason I can think of — and then it also permits a suitably sadomasochistic programmer to define his own specialization of std::allocator_traits without causing compiler errors in <vector>.
I have added a test case in test/libcxx/ for the sadomasochistic case, which I describe as "arguably ill-formed." I would be very very happy to see WG21 agree that specializing traits classes (pointer_traits, allocator_traits, iterator_traits) *is* ill-formed; I believe there's some disagreement on the subject at the moment. In the meantime, I think this would be a nice patch just on code-cleanliness grounds.
This patch is also groundwork for the "trivially relocatable" fork that I'm building on my GitHub; we'd need an architecture something like this in order to easily drop in support for trivial relocatability.
UPDATE: I suppose I should point out for context that a draft of the "trivially relocatable" proposal is now public, and a fork of libc++ incorporating the proposed feature is available on Compiler Explorer with vastly improved codegen compared to vanilla libc++.