Proposed resolution:

This wording is relative to N4835.

1. Modify [allocator.members] as indicated:

   [[nodiscard]] constexpr T* allocate(size_t n);
   

   -?- Mandates: T is not an incomplete type ([basic.types]).

   -2- Returns: A pointer to the initial element of an array of storage of size n * sizeof(T), aligned appropriately for objects of type T.

   […]

[ 2019-11 Status to 'Ready' in Monday issue prioritization in Belfast ]

In C++20 the std::allocator<void> explicit specialization is gone, which means it uses the primary template, which has allocate and deallocate members.

Although it's implied by the use of sizeof(T), std::allocator<T>::allocate doesn't have an explicit precondition that the value type is complete.