A timer for arbitrary functions

Question

I tried to build a function template that can measure the execution time of functions of arbitrary type. Here is what I've tried so far:

#include <chrono>
#include <iostream>
#include <type_traits>
#include <utility>

// Executes fn with arguments args and returns the time needed
// and the result of f if it is not void
template <class Fn, class... Args>
auto timer(Fn fn, Args... args)
    -> std::pair<double, decltype(fn(args...))> {
  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>
double timer_void(Fn fn, Args... args) {
  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;
}

Notice that I have to have a function with a different name for void(...) functions. Is there any way to get rid of the second function?

(And is what I did correct in the first place?)