4204. specification of as-sndr2(Sig) in [exec.let] is incomplete

Section: 33.9.12.10 [exec.let] Status: WP Submitter: Eric Niebler Opened: 2025-02-03 Last modified: 2025-06-23

Priority: 1

View all other issues in [exec.let].

View all issues with WP status.

Discussion:

33.9.12.10 [exec.let]/p9 reads:

Given a type Tag and a pack Args, let as-sndr2 be an alias template such that as-sndr2<Tag(Args...)> denotes the type call-result-t<Fn, decay_t<Args>&...>. Then ops2_variant_t denotes the type

 variant<monostate, connect_result_t<as-sndr2<LetSigs>, receiver2<Rcvr, Env>>...>

except with duplicate types removed.

Paragraphs 8, 9, and 10 only make sense in relation to the lambda in paragraph 7, but that is not at all clear from the current wording. I suggest making paragraphs 8, 9, and 10 sub-bullets of paragraph 7.

[2025-02-07; Reflector poll]

Set priority to 1 after reflector poll.

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

Dropped the suggestion to nest p8-10 under p7.

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

Proposed resolution:

This wording is relative to N5001.

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

   -7- impls-for<decayed-typeof<let-cpo>>::get-state is initialized with a callable object equivalent to the following:

   
     []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) requires see below {
       auto& [_, fn, child] = sndr;
       using fn_t = decay_t<decltype(fn)>;
       using env_t = decltype(let-env(child));
       using args_variant_t = see below;
       using ops2_variant_t = see below;
   
       struct state-type {
         fn_t fn;                    // exposition only
         env_t env;                  // exposition only
         args_variant_t args;        // exposition only
         ops2_variant_t ops2;        // exposition only
       };
       return state-type{std::forward_like<Sndr>(fn), let-env(child), {}, {}};
     }
   

   -8- Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<child-type<Sndr>, 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.

   -9- Given a type Tag and a pack Args, let as-sndr2 be an alias template such that as-sndr2<Tag(Args...)> denotes the type call-result-t<Fn, decay_t<Args>&...>. Then ops2_variant_t denotes the type

    variant<monostate, connect_result_t<as-sndr2<LetSigs>, receiver2<Rcvr, Envenv_t>>...>
   

   except with duplicate types removed.

   -10- The requires-clause constraining the above lambda is satisfied if and only if the types args_variant_t and ops2_variant_t are well-formed.