Proposed resolution:

This wording is relative to N4849.

1. Modify [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 [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 noexcept_opt for a class type G.

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

   […]

   packaged_task(packaged_task&& rhs) noexcept;
   

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

[ Status to Ready on Friday in Prague. ]

[ 2020-02-14; Prague ]

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

F N A
1 7 6

[ 2020-02-13; Prague ]

LWG improves wording matching Marshall's Mandating paper.

[ 2018-06-23 after reflector discussion ]

Priority set to 3

Previous resolution [SUPERSEDED]:

This wording is relative to N4750.

1. Modify [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 [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 noexcept_opt for a class type G, then the deduced type is packaged_task<R(A...)>.

   […]

   packaged_task(packaged_task&& rhs) noexcept;
   

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.