C++ auto return type

I wrote a function that looks like that:

auto fn(auto x) {
    return x;
}

I called it twice with different arguments:

std::cout << fn(3124) << std::endl;
std::cout << fn("hello world") << std::endl;

It works fine, but I don't understand why - I thought the compiler deduces a constant return type for the function - int (due to the first call - fn(3124)). It seems that the compile-time generated function looks like

template<typename T>;
T fn(T x) {
   return x;
}

I can't figure out why. Please explain.