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### 3117. Missing `packaged_task` deduction guides

Section: 32.9.10 [futures.task] Status: WP Submitter: Marc Mutz Opened: 2018-06-08 Last modified: 2020-11-09

Priority: 3

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Discussion:

`std::function` has deduction guides, but `std::packaged_task`, which is otherwise very similar, does not. This is surprising to users and I can think of no reason for the former to be treated differently from the latter. I therefore propose to add deduction guides for packaged task with the same semantics as the existing ones for function.

[2018-06-23 after reflector discussion]

Priority set to 3

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> 1. Modify 32.9.10 [futures.task], class template `packaged_task` synopsis, as indicated:

>    >     namespace std {
>    >       […]
>    >       template<class R, class... ArgTypes>
>    >       class packaged_task<R(ArgTypes...)> {
>    >         […]
>    >       };

>    >       template<class R, class... ArgTypes>
>    >       packaged_task(R (*)( ArgTypes ...)) -> packaged_task<R( ArgTypes...)>;

>    >       template<class F> packaged_task(F) -> packaged_task<see below>;

>    >       template<class R, class... ArgTypes>
>    >         void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;
>    >     }

> 2. Modify 32.9.10.2 [futures.task.members] as indicated:

>    >     template<class F>
>    >       packaged_task(F&& f);

>    > […]

>    >     template<class F> packaged_task(F) -> packaged_task<see below>;

>    > > -?- Remarks: This deduction guide participates in overload resolution only if `&F::operator()` is well-formed when treated as an unevaluated operand. In that case, if `decltype(&F::operator())` is of the form `R(G::*)(A...) cv &opt noexceptopt` for a class type `G`, then the deduced type is `packaged_task<R(A...)>`.

>    > […]

>    >     packaged_task(packaged_task&& rhs) noexcept;

[2020-02-13; Prague]

LWG improves wording matching Marshall's Mandating paper.

[2020-02-14; Prague]

Do we want a feature test macro for this new feature?

>     F N A
>     1 7 6

[Status to Ready on Friday in Prague.]

[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 32.9.10 [futures.task], class template `packaged_task` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       template<class R, class... ArgTypes>
   >       class packaged_task<R(ArgTypes...)> {
   >         […]
   >       };

   >       template<class R, class... ArgTypes>
   >       packaged_task(R (*)(ArgTypes...)) -> packaged_task<R(ArgTypes...)>;

   >       template<class F> packaged_task(F) -> packaged_task<see below>;

   >       template<class R, class... ArgTypes>
   >         void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;
   >     }

2. Modify 32.9.10.2 [futures.task.members] as indicated:

   >     template<class F>
   >       packaged_task(F&& f);

   > […]

   >     template<class F> packaged_task(F) -> packaged_task<see below>;

   > > -?- Constraints: `&F::operator()` is well-formed when treated as an unevaluated operand and `decltype(&F::operator())` is of the form `R(G::*)(A...) cv &opt noexceptopt` for a class type `G`.

   > > -?- Remarks: The deduced type is `packaged_task<R(A...)>`.

   > […]

   >     packaged_task(packaged_task&& rhs) noexcept;