I am adopting CollectionsTestSupport code for my own testing purpose. I noticed that in checkCollection, distance(from:to:), the testing code seems to get all possible pairs of all indices, rather than making sure that the first index is less or equal to the second one:

I believe the documentation for Collection says:

Unless the collection conforms to the BidirectionalCollection protocol, start must be less than or equal to end.

So this is probably a bug? It causes issues for my non-bidirectional collections.