Function apply

include/exec/libdispatch_queue.hpp:408–413

      template <class F>
      void apply(F f)
      {
        std::visit([&](auto &tupl) -> void
                   { std::apply([&](auto &...args) -> void { f(args...); }, tupl); },
                   data_);
      }

      variant_t data_;
      Receiver  rcvr_;