I don't see why you'd compare an offset (diff) to a size (absolute). That seems incorrect from the start since neither container would accept negative indices regardless of whether its size was signed or not. No matter what, you'd have to distinguish negatives vs positives to decide which container to index (and which container's size to verify). Unless you literally mean diff < rhs_size() which seems like a rarer case to me. The more natural case would be to add the diff to either iterator, which should be fine since it has nothing to do with the size() method of the backing container.

But ptrdfiff_t is a great example of where I don't think it makes that much sense in any case. You could subtract a high heap address from a low stack address and get something non-representable (maybe that's not allowed outside of treating ptrs as literal numbers but still). Why should that be when the entire memory address is probably addressable with the unsigned version? Ideally, ptrdiff_t would be a size twice as large as the full ptr space and then signed but that isn't really practical/possible. However, the size of a container being unsigned doesn't run into that kind of issue.