async_write.cpp

template <typename BufferSequence>
auto
async_write (tcp::socket& socket, BufferSequence const& buffers) {
    struct Awaitable {
        tcp::socket& socket;
        BufferSequence const& buffers;
        boost::system::error_code ec;

        bool
        await_ready () {
            return false;
        }

        void
        await_suspend (std::experimental::coroutine_handle<> coro) {
            return asio::async_write (
                socket,
                buffers,
                [this, coro](auto ec, std::size_t bytes_written) mutable {
                    if (ec) {
                        this->ec = ec;
                    }
                    coro.resume ();
                });
        }

        auto
        await_resume () {
            return ec;
        }
    };

    return Awaitable{socket, buffers};
};