c++ - <unresolved overloaded function type>

In my class called Mat, I want to have a function which takes another function as a parameter. Right now I have the 4 functions below, but I get an error in when calling print(). The second line gives me an error, but I don't understand why, since the first one works. The only difference is function f is not a member of the class Mat, but f2 is. The failure is: error: no matching function for call to Mat::test( < unresolved overloaded function type>, int)'

template <typename F>
int Mat::test(F f, int v){
    return f(v);
}

int Mat::f2(int x){
    return x*x;
}

int f(int x){
    return x*x;
}

void Mat::print(){
    printf("%d\n",test(f ,5));    // works
    printf("%d\n",test(f2 ,5));    // does not work
}

Why does this happen?