@swift-ci create

Progress so far:

• [Stdlib] Use associated type where clauses to address various ABI FIXMEs. #8593

• [Stdlib] For Collections, the SubSequence of a Subsequence is SubSequence #8888

• [Stdlib] For Collections, the SubSequence of a Subsequence is SubSequence #8888

• [stdlib] Add Collection constraints via protocol where clauses #9374

Several of these were addressed by #11769

• Sequence.SubSequence must be a Sequence

• Sequence.SubSequence.Element must be same type as Sequence.Element

#11918 adds:

• Collection.Indices should be a Collection

• BidirectionalCollection.Indices should be a BidirectionalCollection

• RandomAccessCollection.Indices should be a RandomAccessCollection

• Collection.Indices.Index should be same type as Collection.Index

#11899 adds:

• Collection.SubSequence should be a Collection

• BidirectionalCollection.SubSequence should be a BidirectionalCollection

• RandomAccessCollection.SubSequence should be a RandomAccessCollection

• MutableCollection.SubSequence should be a MutableCollection

• RangeReplaceableCollection.SubSequence should be a RangeReplaceableCollection

I believe that the following need more design discussion and should be split out into a separate proposal if we're going to do them:

• Iterator should be a Sequence

• Collection.SubSequence.Indices should be same type as Collection.Indices

Everything else listed above was already taken care of by @airspeedswift's work. The second of them, for example, means that all of the ``Default*Indices`` types need to use

public typealias Indices = Base.Indices

Which feels a little awkward.

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