1 Introduction

The paper [P2999R3] “Sender Algorithm Customization” proposed, among other things, to make sender algorithms customizable lazily; that is, when their senders are connected with receivers. LEWG agreed and forwarded P2999 to LWG. Due to a gross oversight, however, P2999 didn’t propose the wording changes to connect and get_completion_signatures that actually implement the design that LEWG approved. This paper corrects the oversight by using the new transform_sender utility in connect and get_completion_signatures as P2999 promised and failed to do.

2 Executive Summary

The changes this paper proposes are:

1. get_completion_signatures(sndr, env) first transforms sndr with transform_sender(get-domain-late(sndr, env), sndr, env) and then uses the result in place of sndr.

2. connect(sndr, rcvr) first transforms sndr with transform_sender(get-domain-late(sndr, get_env(rcvr)), sndr, get_env(rcvr)) and then uses the result in place of sndr.

3 Discussion

Table 2 in [P2999R3] shows how that paper proposed to change the connect customization point. The table is reproduced below. Note that P2999R3 was targetting a version of P2300 that still employed tag_invoke, which has since been removed.

struct connect_t {
  template <receiver Receiver, sender_in<env_of_t<Receiver>> Sender>
    requires /* ... */
  auto operator()(Sender&& snd, Receiver&& rcv) const {






    // First, look for a customization of tag_invoke:
    if constexpr (tag_invocable<connect_t, Sender, Receiver>) {
      return tag_invoke(*this,
                        std::forward<Sender>(snd),
                        std::forward<Receiver>(rcv));
    }
    // Next, see if the sender is co_await-able:
    else if constexpr (is-await-connectable<Sender, Receiver>) {
      /* ... */
    }
  }
};

struct connect_t {
  template <receiver Receiver, sender_in<env_of_t<Receiver>> Sender>
    requires /* ... */
  auto operator()(Sender&& snd, Receiver&& rcv) const {
    // Apply any sender tranformations using the receiver's domain:
    auto&& snd2 = transform_sender(get-domain-late(snd, get_env(rcv)),
                                   std::forward<Sender>(snd),
                                   get_env(rcv));
    using Sender2 = decltype(snd2);

    // First, look for a customization of tag_invoke:
    if constexpr (tag_invocable<connect_t, Sender2, Receiver>) {
      return tag_invoke(*this,
                        std::forward<Sender2>(snd2),
                        std::forward<Receiver>(rcv));
    }
    // Next, see if the sender is co_await-able:
    else if constexpr (is-await-connectable<Sender2, Receiver>) {
      /* ... */
    }
  }
};

The design shown above is the one that was discussed and voted on in LEWG.

A glance at the proposed wording from P2999R3 shows that no such change was ever made. This most critical part of that paper’s design intent was inadvertantly left out of the wording. Face, meet palm.

This paper proposes no design changes from those described in P2999R3. It “merely” corrects the wording to agree with the design.

4 Proposed Wording

[ Editor's note: The changes in this paper are relative to [P2300R9]. ]

[ Editor's note: Change [exec.getcomplsigs] as follows: ]

execution::get_completion_signatures [exec.getcomplsigs]

1. get_completion_signatures is a customization point object. Let sndr be an expression such that decltype((sndr)) is Sndr, and let env be an expression such that decltype((env)) is Env. Let new_sndr be the expression transform_sender(get-domain-late(sndr, env), sndr, env), and let NewSndr be decltype((new_sndr)). Then get_completion_signatures(sndr, env) is expression-equivalent to:

   1. decltype(new_sndr.get_completion_signatures(env)){} if that expression is well-formed,

   2. Otherwise, remove_cvref_t<NewSndr>::completion_signatures{} if that expression is well-formed,

   3. Otherwise, if is-awaitable<NewSndr, env-promise<Env>> is true, then:

      completion_signatures<
        SET-VALUE-SIG(await-result-type<NewSndr, env-promise<Env>>), // see [exec.snd.concepts]
        set_error_t(exception_ptr),
        set_stopped_t()>{}

   4. Otherwise, get_completion_signatures(sndr, env) is ill-formed.

2. Let rcvr be an rvalue receiver of type Rcvr, and let Sndr be the type of a sender such that sender_in<Sndr, env_of_t<Rcvr>> is true. Let Sigs... be the template arguments of the completion_signatures specialization named by completion_signatures_of_t<Sndr, env_of_t<Rcvr>>. Let CSO be a completion function. If sender Sndr or its operation state cause the expression CSO(rcvr, args...) to be potentially evaluated ([basic.def.odr]) then there shall be a signature Sig in Sigs... such that MATCHING-SIG(decayed-typeof<CSO>(decltype(args)...), Sig) is true ([exec.general]).

[ Editor's note: Change [exec.connect] paragraphs 2 and 6 as shown below. Paragraphs 3-5 are unchanged but are shown here in their entirety to give context to the surrounding changes. ]

execution::connect [exec.connect]

1. connect connects ([async.ops]) a sender with a receiver.

2. The name connect denotes a customization point object. For subexpressions sndr and rcvr, let Sndr be decltype((sndr)) and Rcvr be decltype((rcvr)), let new_sndr be the expression transform_sender(get-domain-late(sndr, get_env(rcvr)), sndr, get_env(rcvr)), let NewSndr be decltype((new_sndr)),, and let DS and DR be the decayed types of NewSndr and Rcvr, respectively.

3. Let connect-awaitable-promise be the following class:

   namespace std::execution {
     struct connect-awaitable-promise
       : with-await-transform<connect-awaitable-promise> {
       DR& rcvr; // exposition only
   
       connect-awaitable-promise(DS&, DR& rcvr) noexcept : rcvr(rcvr) {}
   
       suspend_always initial_suspend() noexcept { return {}; }
       [[noreturn]] suspend_always final_suspend() noexcept { terminate(); }
       [[noreturn]] void unhandled_exception() noexcept { terminate(); }
       [[noreturn]] void return_void() noexcept { terminate(); }
   
       coroutine_handle<> unhandled_stopped() noexcept {
         set_stopped((DR&&) rcvr);
         return noop_coroutine();
       }
   
       operation-state-task get_return_object() noexcept {
         return operation-state-task{
           coroutine_handle<connect-awaitable-promise>::from_promise(*this)};
       }
   
       env_of_t<const DR&> get_env() const noexcept {
         return execution::get_env(rcvr);
       }
     };
   }

4. Let operation-state-task be the following class:

   namespace std::execution {
     struct operation-state-task {
       using operation_state_concept = operation_state_t;
       using promise_type = connect-awaitable-promise;
       coroutine_handle<> coro; // exposition only
   
       explicit operation-state-task(coroutine_handle<> h) noexcept : coro(h) {}
       operation-state-task(operation-state-task&& o) noexcept
         : coro(exchange(o.coro, {})) {}
       ~operation-state-task() { if (coro) coro.destroy(); }
   
       void start() & noexcept {
         coro.resume();
       }
     };
   }

5. Let V name the type await-result-type<DS, connect-awaitable-promise>, let Sigs name the type:

   completion_signatures<
     SET-VALUE-SIG(V), // see [exec.snd.concepts]
     set_error_t(exception_ptr),
     set_stopped_t()>

   and let connect-awaitable be an exposition-only coroutine defined as follows:

   namespace std::execution {
     template<class Fun, class... Ts>
     auto suspend-complete(Fun fun, Ts&&... as) noexcept { // exposition only
       auto fn = [&, fun]() noexcept { fun(std::forward<Ts>(as)...); };
   
       struct awaiter {
         decltype(fn) fn;
   
         static constexpr bool await_ready() noexcept { return false; }
         void await_suspend(coroutine_handle<>) noexcept { fn(); }
         [[noreturn]] void await_resume() noexcept { unreachable(); }
       };
       return awaiter{fn};
     };
   
     operation-state-task connect-awaitable(DS sndr, DR rcvr) requires receiver_of<DR, Sigs> {
       exception_ptr ep;
       try {
         if constexpr (same_as<V, void>) {
           co_await std::move(sndr);
           co_await suspend-complete(set_value, std::move(rcvr));
         } else {
           co_await suspend-complete(set_value, std::move(rcvr), co_await std::move(sndr));
         }
       } catch(...) {
         ep = current_exception();
       }
       co_await suspend-complete(set_error, std::move(rcvr), std::move(ep));
     }
   }

6. If Sndr does not satisfy sender or if Rcvr does not satisfy receiver, connect(sndr, rcvr) is ill-formed. Otherwise, the expression connect(sndr, rcvr) is expression-equivalent to:

   1. new_sndr.connect(rcvr) if that expression is well-formed.

      • Mandates: The type of the expression above satisfies operation_state.

   2. Otherwise, connect-awaitable(new_sndr, rcvr) if that expression is well-formed.

   3. Otherwise, connect(sndr, rcvr) is ill-formed.

5 References