#include <type_traits>
#include <iostream>
#include <chrono>
using namespace std;

template <class Fn, class... Args>
auto timer(Fn fn, Args && ... args)
    -> typename std::enable_if< 
    	!std::is_same< decltype( fn( std::forward<Args>(args) ... )), void >::value,
    	std::pair<double, decltype(fn(args...))> >::type
{
  static_assert(!std::is_void<decltype(fn(args...))>::value,
                "Call timer_void if return type is void!");
  auto start = std::chrono::high_resolution_clock::now();
  auto ret = fn(args...);
  auto end = std::chrono::high_resolution_clock::now();
  std::chrono::duration<double> elapsed_seconds = end - start;
  return { elapsed_seconds.count(), ret };
}

// If fn returns void, only the time is returned
template <class Fn, class ... Args>
auto timer(Fn fn, Args && ... args) -> typename std::enable_if< 
    	std::is_same< decltype( fn( std::forward<Args>(args) ... )), void >::value,
    	double>::type
{
  static_assert(std::is_void<decltype(fn(args...))>::value,
                "Call timer for non void return type");
  auto start = std::chrono::high_resolution_clock::now();
  fn(args...);
  auto end = std::chrono::high_resolution_clock::now();
  std::chrono::duration<double> elapsed_seconds = end - start;
  return elapsed_seconds.count();
}



int main () {
    //This call is ambigous if the templates have the same name
    std::cout << timer([](double a, double b){return a*b;},1,2).first;
}