How to `decltype` a functors operator()?

I wanna build a simple templated factory-function, which just gets a functor as template parameter and the functors arguments as variables.

Below some code. The problem is the templated function. When compiling I got:

$ clang++ -std=c++14 test.cpp -o test
test.cpp:32:16: error: no matching function for call to 'factory'
  std::cout << factory<variance_product_producer>(1.0) << std::endl;
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:24:28: note: candidate template ignored: substitution failure [with T = variance_product_producer]: call
      to non-static member function without an object argument
decltype(&T::operator()()) factory(Args... args) {
             ~~~~~~~~      ^
1 error generated.

What should I change to make it run? The code:

struct product
{
  int i = 0;
};

struct complex_product_producer
{
  complex_product_producer(float _setting)
  : setting_(_setting)
  {
  }

  product operator()()
  {
    product p;
    p.i = 10;
    return p;
  }

  float setting_;
};

template<typename T, typename... Args>
decltype(&T::operator()()) factory(Args... args) {
  T t(args...);
  return t();
}

#include <iostream>
int main()
{
  std::cout << factory<complex_product_producer>(1.0) << std::endl;
}