Finding out the return type of a function, lambda or function

This seems to be solved for the case of lambdas in this question. But it is a 2011 answer and I'm looking for a general case: lambdas, regular functions, and functors. And, if possible, by means of the most modern c++ language additions. (Note: g++ -std=c++1y test.cpp).

So, given a function (or a lambda), I'm trying to find out what's its return type. For instance, to declare a variable (simplified case).

int h (int i)  { 
  return 2*i; 
}

int main () {
  // any of the following should be equivalent to int a; int b; int c;
  decltype(h) a; // <<<<< wrong, a's type is not int
  result_of(h) b; // <<<<<< wrong, doesn't compile
  result_of<decltype(h)> c; // <<<<<< wrong c's type is not int
}

Thanks.