Open Radar

 
Summary:
It is impossible to tell if one ForwardIndex or BidirectionalIndex comes before or after another. This makes certain algorithms quite difficult to write.

Steps to Reproduce:
1. Try to implement this function in a playground. It should return the distance, whether positive or negative, between two indexes:

    func - <IndexType: BidirectionalIndex>(lhs: IndexType, rhs: IndexType) -> IndexType.DistanceType

Expected Results:
You can test whether lhs < rhs or rhs < lhs and then proceed accordingly.

Actual Results:
You cannot. I have not been able to find a way to implement this function.

Notes:
You also can’t implement this function by iterating in both directions, because you can’t get the predecessor of the startIndex, nor (presumably) the successor of the endIndex. That limitation is a bit more understandable, though.