If I run the following code, I get different addresses printed. Why?

class Base1 {
    int x;
};

class Base2 {
    int y;
};

class Derived : public Base1, public Base2 {

};

union U {
    Base2* b;
    Derived* d;
    U(Base2* b2) : b(b) {}
};

int main()
{
    Derived* d = new Derived;

    cout << d << "\n";
    cout << U(d).d << "\n";

    return 0;
}

Even more fun is if you repeatedly go in and out of the union the address keeps incrementing by 4, like this

int main()
{
    Derived* d = new Derived;

    cout << d << "\n";
    d = U(d).d;
    cout << d << "\n";
    d = U(d).d;
    cout << d << "\n";

    return 0;
}

If the union is modified like this, then the problem goes away

union U {
    void* v;
    Base2* b;
    Derived* d;
    U(void* v) : v(v) {}
};

Also, if either base class is made empty, the problem goes away. Is this a compiler bug? I want it to leave my pointers the hell alone.