using coro::future< T >::cast_ret_value = std::conditional_t<std::is_void_v<T>, bool, std::add_rvalue_reference_t<T> >
type which is used for cast operator ()
This is always T && (if T is reference, then T & && -> T &) The type is bool in case that T is void, because in this case, bool is actually used as storage without defined value
