deduction guides for std::packaged_task

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

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

1) This deduction guide is provided for std::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 and decltype(&F::operator()) is of the form R(G::*)(A...) (optionally cv-qualified, optionally noexcept, optionally lvalue reference qualified) for some class type G. 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; }; // deduces packaged_task<int(double)>
}