std::move() silently falls back to copying if it cannot move. This can
cause unexpected slowdown which is expensive to pinpoint.


I propose std::must_move(), with deleted overloads for const T& and
const T&&. This allows the user to request a diagnostic instead of
falling back to a copy.


Note that a copy will still happen if the type does not have a move
constructor, so the proposal only prevents errors at the call site.


Possible implementation:


template <typename T>

T&& must_move(T& x) {

     return static_cast<T&&>(x);

}


template <typename T>

T&& must_move(T&& x) {

     return static_cast<T&&>(x);

}


template <typename T>

T&& must_move(const T&) = delete;


template <typename T>

T&& must_move(const T&&) = delete;