if ((v1.x()-v2.x())*(v1.x()-v2.x())+(v1.y()-v2.y())*(v1.y())-v2.y())+(v1.z()-v2.z())*(v1.z()-v2.z()) > distance*distance) {
        // collision
    }

Would you like this to fail to compile? Or would you prefer it to silently return the wrong value?