4528. task needs get_completion_signatures()

Section: 33.13.6.2 [task.class] Status: New Submitter: Dietmar Kühl Opened: 2026-02-21 Last modified: 2026-02-22

Priority: Not Prioritized

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Discussion:

The changes made by P3164 mean that it isn't sufficient to have a completion_signatures type alias in the task class. Instead, it needs to have a static consteval get_completion_signatures() member function that returns the completion signatures. Instead of defining a type alias, the function can return the same type.

Proposed resolution:

This wording is relative to N5032.

1. Change the task synopsis in 33.13.6.2 [task.class] to not have a completion_signatures type alias, but instead have a static consteval get_completion_signatures() member function that returns the completion signatures:

   namespace std::execution {
     template<class T = void, class Environment = env<>>
     class task {
       // [task.state]
       template<receiver Rcvr>
         class state;                              // exposition only
   
     public:
       using sender_concept = sender_t;
       using completion_signatures = see below;
       using allocator_type = see below;
       using scheduler_type = see below;
       using stop_source_type = see below;
       using stop_token_type = decltype(declval<stop_source_type>().get_token());
       using error_types = see below;
   
       // [task.promise]
       class promise_type;
   
       task(task&&) noexcept;
       ~task();
   
       template<class Self, class... Env>
         static consteval auto get_completion_signatures();
   
       template<receiver Rcvr>
         state<Rcvr> connect(Rcvr&& rcvr) &&;
   
     private:
       coroutine_handle<promise_type> handle;      // exposition only
     };
   }
   

2. Remove 33.13.6.2 [task.class] paragraph 4 (the specification moves to 33.13.6.3 [task.members]):

   -4- The type alias completion_signatures is a specialization of execution::completion_signatures with the template arguments (in unspecified order):

   -4.1- -- set_value_t() if T is void, and set_value_t(T) otherwise;

   -4.2- -- template arguments of the specialization of execution::completion_signatures denoted by error_types; and

   -4.3- -- set_stopped_t().

3. Add a new paragraph to 33.13.6.3 [task.members] before the specification of the connect member function, with the specification of the new get_completion_signatures member function:

   template<class Self, class... Env>
     static consteval auto get_completion_signatures();
   

   -?- Let the type C be a specialization of execution::completion_signatures with the template arguments (in unspecified order):

   -?.1- -- set_value_t() if T is void, and set_value_t(T) otherwise;

   -?.2- -- template arguments of the specialization of execution::completion_signatures denoted by error_types; and

   -?.3- -- set_stopped_t().

   -?- Returns: C().