4529. task::promise_type::await_transform declaration and definition mismatch

Section: 33.13.6.5 [task.promise] Status: New Submitter: Dietmar Kühl Opened: 2026-02-21 Last modified: 2026-02-22

Priority: Not Prioritized

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Discussion:

The declarations of await_transform mismatch between the synopsis in 33.13.6.5 [task.promise] and the definition in 33.13.6.5 [task.promise] paragraph 9:

    template<class A>
      auto await_transform(A&& a);
    template<class Sch>
      auto await_transform(change_coroutine_scheduler<Sch> sch);

    template<sender Sender>
      auto await_transform(Sender&& sndr) noexcept;
    template<class Sch>
      auto await_transform(change_coroutine_scheduler<Sch> sch) noexcept;

The declaration should be fixed to match the definition.

Proposed resolution:

This wording is relative to N5032.

1. Change the declarations of await_transform in the synopsis of promise_type in 33.13.6.5 [task.promise] to match the definition in 33.13.6.5 [task.promise] p9 except for omitting the noexcept specifier:

   namespace std::execution {
     template<class T, class Environment>
     class task<T, Environment>::promise_type {
     public:
       ... 
       template<class E>
         unspecified yield_value(with_error<E> error);
   
       template<class Asender Sender>
         auto await_transform(A&& aSender&& sndr);
       template<class Sch>
         auto await_transform(change_coroutine_scheduler<Sch> sch);
   
       unspecified get_env() const noexcept;
       ...
     };
   }
   

2. Remove the noexcept specifier from the declarations of await_transform in the definition of promise_type in 33.13.6.5 [task.promise] p9:

   template<sender Sender>
     auto await_transform(Sender&& sndr) noexcept;
   

   -9- Returns: If same_as<inline_scheduler, scheduler_type> is true returns as_awaitable(std::forward<Sender>(sndr), *this); otherwise returns as_awaitable(affine_on(std::forward<Sender>(sndr), SCHED(*this)), *this).

   template<class Sch>
     auto await_transform(change_coroutine_scheduler<Sch> sch) noexcept;
   

   -10- Effects: Equivalent to:

        return await_transform(just(exchange(SCHED(*this), scheduler_type(sch.scheduler))), *this);