[swift-evolution] Shouldn't ".withUnsafeBufferPointer" and ".withUnsafeMutableBufferPointer" be parts of protocols?

[Brent Royal-Gordon]

> On Jan 28, 2017, at 11:48 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
> The way to handle Deque is to add this requirement to Collection when
> we get the language features to express it:
> 
>  protocol Collection {
> 
>    associatedtype Segments : Collection 
>    where Segments.Element : Collection,
>      Segments.Element.Element == Element
>     = EmptyCollection<EmptyCollection<Element>>
> 
>    var segments: Segments? {get}
>    ...
>  }
> 
>  extension Collection 
>  where Segments == EmptyCollection<EmptyCollection<Element>> {
>    var segments: Segments? { return nil }
>  }

Couldn't that be be expressed more accurately with `Never` if it were a subtype-of-all-types?

	 protocol Collection {
	
	   associatedtype Segments : Collection 
	   where Segments.Element : Collection,
	     Segments.Element.Element == Element
	    = Never
	
	   var segments: Segments? {get}
	   ...
	 }
	
	 extension Collection 
	 where Segments == Never {
	   var segments: Segments? { return nil }
	 }

Or you could say that there is always at least *one* segment:

	 protocol Collection {
	
	   associatedtype Segments : Collection 
	   where Segments.Element : Collection,
	     Segments.Element.Element == Element
	    = CollectionOfOne<Self>
	
	   var segments: Segments {get}
	   ...
	 }
	
	 extension Collection 
	 where Segments == CollectionOfOne<Self> {
	   var segments: Segments { return CollectionOfOne(self) }
	 }

-- 
Brent Royal-Gordon
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