sender concepts for awaitable types

[maikel]

I was making my own coroutine task basic_task<Ret, Queries...> which is an awaitable for promises that have an env that is queryable for all specified Queries...

The current spec defines the following two sender concepts

template<class Sndr>
    concept enable-sender =                                     // exposition only
      is-sender<Sndr> ||
      is-awaitable<Sndr, env-promise<empty_env>>;               // [exec.awaitable]

  template<class Sndr>
    concept sender =
      bool(enable-sender<remove_cvref_t<Sndr>>) &&
      requires (const remove_cvref_t<Sndr>& sndr) {
        { get_env(sndr) } -> queryable;
      } &&
      move_constructible<remove_cvref_t<Sndr>> &&
      constructible_from<remove_cvref_t<Sndr>, Sndr>;

  template<class Sndr, class Env = empty_env>
    concept sender_in =
      sender<Sndr> &&
      queryable<Env> &&
      requires (Sndr&& sndr, Env&& env) {
        { get_completion_signatures(std::forward<Sndr>(sndr), std::forward<Env>(env)) }
          -> valid-completion-signatures;
      };

The current concepts require basic_task<Ret, Queries...> to be awaitable in env-promise<empty_env> which is too strict, IMHO. Instead we could require for sender_in:

  template<class Sndr, class Env = empty_env>
    concept sender_in =
     is-awaitable<Sndr, env-promise<Env>> ||
      sender<Sndr> &&
      queryable<Env> &&
      requires (Sndr&& sndr, Env&& env) {
        { get_completion_signatures(std::forward<Sndr>(sndr), std::forward<Env>(env)) }
          -> valid-completion-signatures;
      };

There are some places where we mandate input to satisfy the sender concept. The most used one is for connect(sndr, rcvr) expressions. The current spec states https://eel.is/c++draft/exec.snd#exec.connect-6

The expression connect(sndr, rcvr) is expression-equivalent to:
(6.1)
new_sndr.connect(rcvr) if that expression is well-formed.
Mandates: The type of the expression above satisfies operation_state.
(6.2)
Otherwise, connect-awaitable(new_sndr, rcvr).
Mandates: sender<Sndr> && receiver<Rcvr> is true.

I would like to Mandate receiver<Rcvr> && sender_in<Sndr, env_of_t<Rcvr>>

Any thoughts?

[ericniebler]

a type that satisfies sender_in needs to satisfy sender. otherwise subsumption is broken.

Why do we require awaitables to work with an empty env?

we don't, but we do require an opt-in to satisfy the sender concept. making your type awaitable with an empty environment is one way to opt in. if your awaitable requires an environment, you have two options:

1. give your awaitable type a nested sender_concept typedef, or
2. specialize enable_sender for your awaitable type.

the problem is that somewhere along the way, the enable_sender variable template was turned into an exposition-only enable-sender concept. we should roll back that change.

[maikel]

Thank you for clarification!

[jwakely]

Created https://cplusplus.github.io/LWG/issue4202

[ericniebler]

I also sent a list of bugs to lwgchair to import, including one for this issue that has a proposed resolution.

#306

[jwakely]

I don't think #306 was one of the ones you asked to be opened as an issue. I'll update LWG 4202 to include that proposed resolution.