String Index unification vs BidirectionalCollection requirements

[masters3d]

@lorentey

If trapping is too much of a potential compatibility problem, then we can have these methods round down to the nearest reachable index instead.

It would be great if the fix aligned Array and String API so that trappings was not needed. I am personally allergic to trapping APIs and much rather deal with optional returns or throwing.

Edit: reference Behavior of index(after:)