make_weak_ref()

template<convertible_to_weak_ref T>
weak_ref_t<T> make_weak_ref(T&& v);

Creates a weak_ref from a convertible_to_weak_ref; for example:

T	make_weak_ref(T)
std::shared_ptr<T>	std::weak_ptr<T>
std::weak_ptr<T>	std::weak_ptr<T>
std::enable_shared_from_this<T>*	std::weak_ptr<T>