33 Execution control library [exec]

33.9 Senders [exec.snd]

33.9.9 execution​::​get_completion_signatures [exec.getcomplsigs]

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template<class Sndr, class... Env> consteval auto get_completion_signatures() -> valid-completion-signatures auto;
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Let except be an rvalue subexpression of an unspecified class type Except such that move_constructible<Except> && derived_from<Except, exception> is true.
Let CHECKED-COMPLSIGS(e) be e if e is a core constant expression whose type satisfies valid-completion-signatures; otherwise, it is the following expression: (e, throw except, completion_signatures())
Let get-complsigs<Sndr, Env...>() be expression-equivalent to remove_reference_t<Sndr>​::​template get_completion_signatures<Sndr, Env...>().
Let NewSndr be Sndr if sizeof...(Env) == 0 is true; otherwise, decltype(s) where s is the following expression: transform_sender( declval<Sndr>(), declval<Env>()...)
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Constraints: sizeof...(Env) <= 1 is true.
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Effects: Equivalent to: return e; where e is expression-equivalent to the following:
  • (3.1)
    CHECKED-COMPLSIGS(get-complsigs<NewSndr, Env...>()) if get-complsigs<NewSndr, Env
    ...>()     is a well-formed expression.
  • (3.2)
    Otherwise, CHECKED-COMPLSIGS(get-complsigs<NewSndr>()) if get-complsigs<NewSndr>() is a well-formed expression.
  • (3.3)
    Otherwise, completion_signatures< SET-VALUE-SIG(await-result-type<NewSndr, env-promise<Env>...>), // [exec.snd.concepts] set_error_t(exception_ptr), set_stopped_t()> if is-awaitable<NewSndr, env-promise<Env>...> is true.
  • (3.4)
    Otherwise, (throw dependent-sender-error(), completion_signatures()) if sizeof...(
    Env) == 0     is true, where dependent-sender-error is dependent_sender_error or an unspecified type derived publicly and unambiguously from dependent_sender_error.
  • (3.5)
    Otherwise, (throw except, completion_signatures()).
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Given a type Env, if completion_signatures_of_t<Sndr> and completion_signatures_of_t<Sndr, Env> are both well-formed, the former shall be a superset of the latter, with the difference corresponding to error or stopped completion operations.
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Let rcvr be an rvalue whose type Rcvr models receiver, 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]).