use_op incorrectly moves lvalue reference arguments passed to async_initiate

[anarthal]

This statement

// ws is a beast::websocket::stream, request is an http::request<http::string_body>
co_await ws.async_accept(request, as_tuple(use_op));

Leaves ws in an invalid state by moving the websocket stream's implementation, which is a shared_ptr passed by lvalue reference to asio::async_initiate.

The problem is in op.hpp, within these lines:

  template <typename Initiation, typename... InitArgs>
  static auto initiate(Initiation && initiation,
                       boost::async::use_op_t,
                       InitArgs &&... args)
      -> op_impl<std::decay_t<Initiation>, std::decay_t<InitArgs>...>
  {
    return op_impl<std::decay_t<Initiation>, std::decay_t<InitArgs>...>(
        std::forward<Initiation>(initiation),
        std::forward<InitArgs>(args)...);
  }

This decay will end up moving lvalue references. Comparing this to other Asio's completion tokens, this should be:

  template <typename Initiation, typename... InitArgs>
  static auto initiate(Initiation initiation, boost::async::use_op_t,
                       InitArgs... args) -> op_impl<Initiation, InitArgs...> {
    return op_impl<Initiation, InitArgs...>(std::move(initiation),
                                            std::move(args)...);
  }

With this change, the code works.