As I understand it, that's precisely what std::start_lifetime_as<T>() does: it effectively performs a placement new to create a T object, except that it retains the existing bytes at the address. It only works with implicit-lifetime types (i.e., scalars, or classes with a trivial constructor), though, so it probably wouldn't work with your hash table example, except perhaps for an inline hash table.
Superb! Looking through https://en.cppreference.com/w/cpp/memory/start_lifetime_as, this appears to be the right thing. It also has volatile overloads (which it looks like placement new still does not). This doesn't appear to be implemented in libc++ yet but that seems fixable, it'll go down the same object construction logic placement new does. Thank you for the reference, that'll fix some ugly edge cases in one of my libraries.
