Subtraction of two nullptr values guaranteed to be zero?

Is it guaranteed by the C++ standard that if I have two pointers of the same type whose value is equal to nullptr, that the difference between those pointers is equal to 0?

In a pseudo-mathematical notation, does the following predicate hold true?

ForAll x ForAll y (x == nullptr)^(y == nullptr) -> (x - y == 0)

The simplest code example I can think of being:

int* x = nullptr;
int* y = nullptr;
assert(x - y == 0);

I suppose this boils down to: is it possible to have a valid implementation of the C++ standard for which there are multiple bit representations of nullptr that only compare as being equal because the equality operator does some magic?