When two type parameters are different, how to choose a proper return type in C++ ?

auto-return-type.cxx

#include <iostream>
#include <typeinfo>

template<typename T, typename S>
auto max(T a, S b) -> decltype(a+b) { // note that T and S may be different.
    // for numeric type, automatic type convension will be performed when compare two values.
    if (a > b) {
        return a;
    }
    else {
        return b;
    }
}

int main() {
    short a = 1;
    int b = 2;
    double c = 3;

    std::cout << typeid(max(a, b)).name() << std::endl; // print "i", return type: int
    std::cout << typeid(max(b, c)).name() << std::endl; // print "d", return type: double

    return 0;
}