deduction guides for std::function

template<class R, class... ArgTypes> 
function(R(*)(ArgTypes...)) -> function<R(ArgTypes...)>;

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

1) This deduction guide is provided for std::function to allow deduction from functions.

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

#include <functional>
int func(double) { return 0; }
int main() {
  std::function f{func}; // guide #1 deduces function<int(double)>
  int i = 5;
  std::function g = [&](double) { return i; }; // guide #2 deduces function<int(double)>
}