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4345. task::promise_type::return_value default template parameter

Section: 33.13.6.5 [task.promise] Status: Tentatively Ready Submitter: Dietmar Kühl Opened: 2025-09-01 Last modified: 2025-10-27

Priority: Not Prioritized

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Discussion:

The template parameter V of task::promise_type::return_value doesn't have a default template argument specified. Specifying a default template argument of T would enable use of co_return { ... } which would be consistent with normal return statements. This feature was not discussed in the design paper but based on the LEWG discussion on 2025-08-26 it is considered to be more a bug fix than a new feature.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Proposed resolution:

Add a default template argument of T to the template parameter V of task::promise_type::return_value in the synopsis of 33.13.6.5 [task.promise]:

namespace std::execution {
  template<class T, class Environment>
  class task<T, Environment>::promise_type {
     ...
    template<typename V = T>
    void return_value(V&& value);
    ...
  };

}