misbehaves in presence of void operator&()

[zygoloid]

196:
template <class X, size_t S = sizeof(std::declval< X&>().operator&()) >

This is not selected if operator& exists but returns void. It's also not selected if the call to operator& is ambiguous or is deleted.

(Ultimately, the right fix here is to make std::addressof be constexpr; in Clang, you can implement constexpr_addressof using __builtin_addressof.)