Can you add a test trying to call construct_at on a pointer-to-const? Apparently, the intent is that construct_at can be called on a pointer-to-const pointing to a location of memory that hasn't been initialized yet.
The idea is that const only applies to objects that whose lifetime has begun, so the pointer-to-const doesn't point to a const object yet (well, not until you've in-place-newed it).
Those can be constexpr unconditionally.
Same, constexpr unconditionally.
Jeez, what a pain. Should we consider supporting construct_at in C++17 as an extension?