String Index unification vs BidirectionalCollection requirements

[Nevin]

While I agree that you have highlighted some important issues, I do not see how this claim follows:

@lorentey

TLDR: SE-0180 is fundamentally incompatible with BidirectionalCollection 's stated requirements

The stated requirement that you quote says (paraphrased), “For any index in startIndex ..< endIndex, calling both index(before:) and index(after:) in either order gets you back to the same index where you began.”

It does not say anything about unreachable indices, except that if you start from one and take a step forward and a step backward, you end up at the same (unreachable) index. This is exactly how I would expect unreachable indices to behave.

Are you saying that String violates that?

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The documentation for Collection includes this passage:

You can pass only valid indices to collection operations. You can find a complete set of a collection's valid indices by starting with the collection's startIndex property and finding every successor up to, and including, the endIndex property. All other values of the Index type, such as the startIndex property of a different collection, are invalid indices for this collection.

That is an explicit statement that unreachable indices are emphatically not valid. Furthermore, the documentation for index(after:), distance(from:to:) and other such operations all include the parameter description:

i: A valid index of the collection.

So the behavior you describe is in fact the direct result of a programmer violating the documented requirements of the methods they are calling.

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The behavior of SE–180 is also quite clear: subscripting with an unreachable (and hence invalid) index is handled by rounding to a valid index.