`long` and `double` are same when converting from `int ` by compiler

I was studying function overloading in c++, and I saw an ambiguous condition, in the program,

long add(long a){
    long b = a;
    return b;
}

double add(double a){
    double b = a;
    return b;
}

int main(){
    int x;
    x = add(10);
    printf("x : %d", x);
    getch();
    return 0;   
}

The reason of ambiguity given in the book was, the compiler may convert int either in long or in double. So compiler generates an error. I run this and the result was same, error.

So how long and double are equivalent? what's the actual reason of ambiguity here?

It is not that long and double are equivalent in themselves, but rather the conversion rules int->long and int->double have equivalent priority, so the compiler faces an ambiguity in the presence of both options.