I noticed that the new Collection index(_:offsetBy:)
<http://swiftdoc.org/v3.0/protocol/Collection/#comment-func-index_offsetby_>
does not define that negative offsets require a BidirectionalCollection. It
also declares that negative offsets must complete in O( | offset | ) time. This
differs from other methods such as distance(from:to:)
<http://swiftdoc.org/v3.0/protocol/Collection/#comment-func-distance-from_to_>which
indicates start <= end if not a BidirectionalCollection
This would preclude some data structures from strictly implementing the
Collection protocol, such as singly-linked lists.
Would it be appropriate to indicate instead that BidirectionalCollection
defines the negative offset behavior and negative offset performance constraint?
-DW
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