[SR-3444] map, filter on RangeReplaceableCollection should return Self

Previous ID	SR-3444
Radar	None
Original Reporter	@airspeedswift
Type	Improvement

Votes	1
Component/s	Standard Library
Labels	Improvement, StarterProposal, swift-evolution-proposal-needed
Assignee	None
Priority	Medium

md5: 05669e9e4e1c642618be3a82e1ececb1

Issue Description:

Currently, map and filter on Sequence return an Array, even though the implementation use only capabilities of Array that are available on RangeReplaceableCollection* (an empty init, reserveCapacity, and append).

This means that RangeReplaceableCollection could implement a version of map that returned the same type. For example, a Deque could return another Deque. This is probably what most users would expect for these types.

This would only apply to RRCs that could hold the mapped-to element type. Also note that, in the case of map, this would only apply to transformations that return the same element type – to represent a transformation to another type would require higher-kinder types. And sequences and non-RRC collections, like Set would need to return arrays.

*(this isn't completely true: it creates a ContiguousArray and then turns it into an Array. But hopefully this optimization could be removed at some point)