Simplest way to determine return type of function

Question

Given a very simple, but lengthy function, such as:

int foo(int a, int b, int c, int d) {
    return 1;
}

// using ReturnTypeOfFoo = ???

W

Answer 1

I don't know if is the simplest way (if you can use C++17 surely isn't: see NathanOliver's answer) but... what about declaring a function as follows:

template 
R getRetType (R(*)(Args...));

and using decltype()?

using ReturnTypeOfFoo = decltype( getRetType(&foo) );

Observe that getRetType() is only declared and not defined because is called only a decltype(), so only the returned type is relevant.