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4203. Constraints on get-state functions are incorrect

Section: 33.9.12.7 [exec.schedule.from] 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 #313 and cplusplus/sender-receiver #314.

33.9.12.7 [exec.schedule.from] p6 reads:

The member impls-for<schedule_from_t>::get-state is initialized with a callable object equivalent to the following lambda: 
  []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(see below)
     requires sender_in<child-type<Sndr>, env_of_t<Rcvr>> {
The constraint should account for the fact that the child sender will be connected with FWD-ENV(get_env(rcvr)).

[ The resolution touches some of the same text as LWG 4198(i), but without conflicting. ]

Imported from: cplusplus/sender-receiver #315.

33.9.12.12 [exec.when.all] p6 reads:

The member impls-for<when_all_t>::get-env is initialized with a callable object equivalent to the following lambda expression: 
  []<class State, class Rcvr>(auto&&, State& state, const Receiver& rcvr) noexcept {
    return see below;
  }
Returns an object e such that
  1. (6.1) — decltype(e) models queryable, and
  2. (6.2) — e.query(get_stop_token) is expression-equivalent to state.stop-src.get_token(), and
  3. (6.3) — given a query object q with type other than cv stop_token_t, e.query(q) is expression-equivalent to get_env(rcvr).query(q).
The problem is in (6.3). It should be forwarding on forwarding-query's to get_env(rcvr) but is is instead forwarding all queries.

Imported from: cplusplus/sender-receiver #316.

The child senders should only see the parent's queries if they are forwarding queries.

Imported from: cplusplus/sender-receiver #311.

33.9.12.14 [exec.stopped.opt]/para 3 reads:

Let sndr and env be subexpressions such that Sndr is decltype((sndr)) and Env is decltype((env)). If sender-for<Sndr, stopped_as_optional_t> is false, or if the type single-sender-value-type<Sndr, Env> is ill-formed or void, then the expression stopped_as_optional.transform_sender(sndr, env) is ill-formed; otherwise, it is equivalent to:
the test for single-sender-value-type<Sndr, Env> is incorrect. It should be testing its child for single-sender-ness.

In addition, it should be applying FWD-ENV-T to Env so that only forwarding queries are forwarded.

[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 33.9.2 [exec.snd.expos] as indicated:
    -2- For a queryable object env, FWD-ENV(env) is an expression whose type satisfies queryable such that for a query object q and a pack of subexpressions as, the expression FWD-ENV(env).query(q, as...) is ill-formed if forwarding_query(q) is false; otherwise, it is expression-equivalent to env.query(q, as...). The type FWD-ENV-T(Env) is decltype(FWD-ENV(declval<Env>())).
  2. Change 33.9.12.7 [exec.schedule.from] as indicated:
    -6- The member impls-for<schedule_from_t>::get-state is initialized with a callable object equivalent to the following lambda: 
      []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(see below)
   requires sender_in<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)> {

    …

    -8- Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)>. Let as-tuple be an alias template that transforms a completion signature Tag(Args...) into the tuple specialization decayed-tuple<Tag, Args...>. Then variant_t denotes the type variant<monostate, as-tuple<Sigs>...>, except with duplicate types removed.

  3. Change 33.9.12.10 [exec.let] as indicated:

    -6- Let receiver2 denote the following exposition-only class template: 

      namespace std::execution {
    …
  }
    Invocation of the function receiver2::get_env returns an object e such that
    1. (6.1) — decltype(e) models queryable and
    2. (6.2) — given a query object q, the expression e.query(q) is expression-equivalent to env.query(q) if that expression is valid,; otherwise, if the type of q satisfies forwarding-query, e.query(q) is expression-equivalent to get_env(rcvr).query(q) ; otherwise, e.query(q) is ill-formed.

    -7- impls-for<decayed-typeof<let-cpo>>::get-state is initialized with a callable object […]

    -8- Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)>. Let LetSigs be a pack of those types in Sigs with a return type of decayed-typeof<set-cpo>. Let as-tuple be an alias template such that as-tuple<Tag(Args...)> denotes the type decayed-tuple<Args...>. Then args_variant_t denotes the type variant<monostate, as-tuple<LetSigs>...> except with duplicate types removed.

  4. Change 33.9.12.12 [exec.when.all] as indicated:

    -6- The member impls-for<when_all_t>::get-env is initialized with a callable object equivalent to the following lambda expression: 

      []<class State, class Rcvr>(auto&&, State& state, const Receiver& rcvr) noexcept {
    return see below;
  }
    Returns an object e such that
    1. (6.1) — decltype(e) models queryable, and
    2. (6.2) — e.query(get_stop_token) is expression-equivalent to state.stop-src.get_token(), and
    3. (6.3) — given a query object q with type other than cv stop_token_t and whose type satisfies forwarding-query, e.query(q) is expression-equivalent to get_env(rcvr).query(q).

    -7- The member impls-for<when_all_t>::get-state is initialized with a callable object equivalent to the following lambda expression: 

      []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(e) -> decltype(e) {
    return e;
  }
    where e is the expression 
      std::forward<Sndr>(sndr).apply(make-state<Rcvr>())
    and make-state is the following exposition-only class template: 
      template<class Sndr, class Env>
  concept max-1-sender-in = sender_in<Sndr, Env> && // exposition only
    (tuple_size_v<value_types_of_t<Sndr, Env, tuple, tuple>> <= 1);

  enum class disposition { started, error, stopped }; // exposition only

  template<class Rcvr>
  struct make-state {
   template<max-1-sender-in<FWD-ENV-T(env_of_t<Rcvr>)>... Sndrs>
    …

    -8- Let copy_fail be exception_ptr if […]

    -9- The alias values_tuple denotes the type 

      tuple<value_types_of_t<Sndrs, FWD-ENV-T(env_of_t<Rcvr>), decayed-tuple, optional>...>
    if that type is well-formed; otherwise, tuple<>.

  5. Change 33.9.12.13 [exec.into.variant] as indicated:
    -5- The member impls-for<into_variant_t>::get-state is initialized with a callable object equivalent to the following lambda: 
      []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept
    -> type_identity<value_types_of_t<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)>> {
    return {};
  }
  6. Change 33.9.12.14 [exec.stopped.opt] as indicated:
    -3- Let sndr and env be subexpressions such that Sndr is decltype((sndr)) and Env is decltype((env)). If sender-for<Sndr, stopped_as_optional_t> is false, or if the type single-sender-value-type<child-type<Sndr>, FWD-ENV-T(Env)> is ill-formed or void, then the expression stopped_as_optional.transform_sender(sndr, env) is ill-formed; otherwise, it is equivalent to: 
      auto&& [_, _, child] = sndr;
  using V = single-sender-value-type<child-type<Sndr>, FWD-ENV-T(Env)>;
    …