Effects: Constructs a move_sentinel, value-initializing last. If is_trivially_default_constructible<S>::value is true, then this constructor is a constexpr constructor.
Effects: Constructs a move_sentinel, initializing last with s.
move_sentinel(const move_sentinel<ConvertibleTo<S>>& s);
Effects: Constructs a move_sentinel, initializing last with s.last.