[SR-13937] Needless precondition in Collection distance

[dabrahams]

Previous ID	SR-13937
Radar	None
Original Reporter	@dabrahams
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug
Assignee	None
Priority	Medium

md5: 79626c31d137f57975f0b894a587bfcd

Issue Description:

The precondition in Collection.distance that reads “Only BidirectionalCollections can have end come before start” fires even for {{BidirectionalCollection}} models unless the implementor of the collection is hyper-vigilant and implements their own distance method.

We could solve this problem by adding a default implementation of distance for BidirectionalCollections, but we're already doing a test/branch even on collections that are non-bidirectional, to detect the precondition violation. We might as well just do the right thing in all cases.

#34981 takes a first crack at an implementation.

[dabrahams]

Hm. We can't lift the precondition in documentation without causing existing working forward collections to be broken. However, we could in principle change the implementation so it works properly for bidirectional collections.