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4202. enable-sender should be a variable template

Section: 33.9.3 [exec.snd.concepts] Status: WP Submitter: Eric Niebler Opened: 2025-02-03 Last modified: 2025-06-23

Priority: 1

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Discussion:

Imported from cplusplus/sender-receiver #305 and cplusplus/sender-receiver #306.

We 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:

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.

[2025-02-07; Reflector poll]

Set priority to 1 after reflector poll.

[Hagenberg 2025-02-11; move to Ready]

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5001.

  1. Change 33.9.3 [exec.snd.concepts] as indicated: 
    
  template<class Sndr>
    concept is-sender =                                         // exposition only
      derived_from<typename Sndr::sender_concept, sender_t>;

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

  template<class Sndr>
    inline constexpr bool enable_sender = enable-sender<Sndr>;

  template<class Sndr>
    concept sender =
      bool(enable-senderenable_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>;

    …

    -2- Given a subexpression sndr, […]

    -?- Remarks: Pursuant to 16.4.5.2.1 [namespace.std], users may specialize enable_sender to true for cv-unqualified program-defined types that model sender, and false for types that do not. Such specializations shall be usable in constant expressions (7.7 [expr.const]) and have type const bool.