1) Thisdeduction guide is provided forstd::packaged_task to allow deduction from functions.

2) This overload participates in overload resolution only if&F::operator() is well-formed when treated as an unevaluated operand anddecltype(&F::operator()) is of the formR(G::*)(A...) (optionally cv-qualified, optionally noexcept, optionally lvalue reference qualified). The deduced type isstd::packaged_task<R(A...)>.

3) This overload participates in overload resolution only if&F::operator() is well-formed when treated as an unevaluated operand andF::operator() is anexplicit object parameter function whose type is of formR(G, A...) orR(G, A...)noexcept. The deduced type isstd::packaged_task<R(A...)>.

4) This overload participates in overload resolution only if&F::operator() is well-formed when treated as an unevaluated operand andF::operator() is astatic member function whose type is of formR(A...) orR(A...)noexcept. The deduced type isstd::packaged_task<R(A...)>.

#include <future> int func(double){return0;} int main(){std::packaged_task f{func};// deduces packaged_task<int(double)> int i=5;std::packaged_task g=[&](double){return i;};// => packaged_task<int(double)>}