Couldn't they also be equal to each other? If equality was defined only in terms of the value and not the vtable (to borrow pcwalton's terms), they would be equal. Are there any problems which would arise from this?
Equality is not necessarily defined for values of different types ("with different vtables", if you will). You can't just compare values of different type and expect a meaningful result.
In this case, though, we're talking about interfaces; an interface is a pointer plus this bit we're calling a vtable. Pointers, even to different types, absolutely can be compared for equality, can't they? More specifically, i am pretty sure pointers can be compared to see if they are both nil, which is all that would be needed here.
