Method __get_completion_signatures

include/exec/unless_stop_requested.hpp:93–98

    {
      template <class _Self, class _Env>
      static consteval auto __get_completion_signatures()
      {
        static_assert(sender_for<_Self, unless_stop_requested_t>);
        // TODO: port this to use constant evaluation
        return __completions_t<__child_of<_Self>, _Env>{};
      };

      template <class _Receiver>
      using __rcvr_t =