Proposed resolution:

This wording is relative to N4892.

1. Modify [func.wrap.func.con] as indicated:

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

   -16- Constraints: &F::operator() is well-formed when treated as an unevaluated operand and decltype(&F::operator()) is either of the form R(G::*)(A...) cv &_opt noexcept_opt or of the form R(*)(G, A...) noexcept_opt for a class type G.

   -17- Remarks: The deduced type is function<R(A...)>.

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

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

   -7- Constraints: &F::operator() is well-formed when treated as an unevaluated operand and decltype(&F::operator()) is either of the form R(G::*)(A...) cv &_opt noexcept_opt or of the form R(*)(G, A...) noexcept_opt for a class type G.

   -8- Remarks: The deduced type is packaged_task<R(A...)>.

[ 2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP. ]

[ 2022-07-15; LWG telecon: move to Ready ]

[ 2022-07-01; Reflector poll ]

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

[ 2021-10-17; Improved wording based on Tim Song's suggestion ]

[ 2021-10-14; Reflector poll ]

Set priority to 2 after reflector poll.

Previous resolution [SUPERSEDED]:

This wording is relative to N4892.

1. Modify [func.wrap.func.con] as indicated:

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

   -16- Constraints: &F::operator() is well-formed when treated as an unevaluated operand and decltype(&F::operator()) is either of the form R(G::*)(A...) cv &_opt noexcept_opt or of the form R(*)(G cv &_opt, A...) noexcept_opt for a class type G.

   -17- Remarks: The deduced type is function<R(A...)>.

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

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

   -7- Constraints: &F::operator() is well-formed when treated as an unevaluated operand and decltype(&F::operator()) is either of the form R(G::*)(A...) cv &_opt noexcept_opt or of the form R(*)(G cv &_opt, A...) noexcept_opt for a class type G.

   -8- Remarks: The deduced type is packaged_task<R(A...)>.

With the adoption of deducing this (P0847), we can now create types whose call operator is an explicit object member function, which means that decltype(&F::operator()) could have pointer-to-function type rather than pointer-to-member-function type. This means that the deduction guides for std::function ([func.wrap.func.con]) and std::packaged_task ([futures.task.members]) will simply fail:

struct F {
  int operator()(this const F&) { return 42; }
};

std::function g = F{}; // error: decltype(&F::operator()) is not of the form R(G::*)(A...) cv &opt noexceptopt

We should update the deduction guides to keep them in line with the core language.