deduction guides for std::packaged_task

1) This deduction guide is provided for std::packaged_task

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

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

4) This overload participates in overload resolution only if &F::operator() is well-formed when treated as an unevaluated operand and F::operator() is a static member function whose type is of form R(A...) or R(A...) noexcept. The deduced type is std::packaged_task <R(A...) >

#include <future>
 
int func(double) { return 0; }
 
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)>
}