Function convert

include/exec/asio/completion_token.hpp:120–123

    template <typename T, typename U>
    constexpr std::remove_cvref_t<T> convert(U&& u)
    {
      return static_cast<U&&>(u);
    }

    template <typename Tuple, typename Receiver, std::size_t... Ns, typename... Args>
    constexpr void set_value_impl(Receiver&& r, std::index_sequence<Ns...>, Args&&... args)