[swift-dev] Associated type inference fun with RandomAccessCollection

[Mark Lacey]

> On Nov 7, 2016, at 6:16 PM, Douglas Gregor via swift-dev <swift-dev at swift.org> wrote:
> 
> Hi all,
> 
> While working on the type checker, I came across an interesting case for associated type inference with the ‘Indices’ type of RandomAccessCollection. At issue is a simple model of RandomAccessCollection where the Index type is Int:
> 
> class ReferenceCollection : RandomAccessCollection {
>   typealias Index = Int
>   
>   var startIndex: Int {
>     return 0
>   }
> 
>   var endIndex: Int {
>     return 1
>   }
> 
>   subscript(index: Int) -> String {
>     return ""
>   }
> 
>   func index(after i: Int) -> Int {
>     return 1
>   }
> 
>   func index(before i: Int) -> Int {
>     return 0
>   }
> }
> 
> What’s the inferred associated Indices? The RandomAccessIterator protocol has a default:
> 
> protocol RandomAccessCollection {
>     associatedtype Indices : _RandomAccessIndexable, BidirectionalCollection
>       = DefaultRandomAccessIndices<Self>
>     var indices: Indices { get }
> }
> 
> which will kick in if nothing else can be inferred. There is also an implementation for this defaulted case in a protocol extension from which we can infer Indices:
> 
> extension RandomAccessCollection where Indices == DefaultRandomAccessIndices<Self> {
>    public var indices: DefaultRandomAccessIndices<Self> { }
> }
> 
> Those line up, which is easy, but there is *another* protocol extension of RandomAccessIterator from which we can infer Indices:
> 
> extension RandomAccessCollection
> where Index : Strideable, 
>       Index.Stride == IndexDistance,
>       Indices == CountableRange<Index> {
> 
>   public var indices: CountableRange<Index> {
>     return startIndex..<endIndex
>   }
> }
> 
> Note that both DefaultRandomAccessIndices<ReferenceCollection> and CountableRange<Int> would be valid inferences for Indices. We have three options:
> 
> 1) Consider type inference to be ambiguous, because there is no natural ordering between the two protocol extensions (they have incompatible same-type constraints on the associated type Indices).
> 2) Consider the first protocol extension to “win” because… we prefer the extension which corresponds to the associated type default (?). This would be consistent with a world where we don’t have associated type inference at all. (It also matches Swift 3.0.1’s behavior).
> 3) Consider the second protocol extension to “win” because…the other protocol extension corresponds to the associated type default, and could therefore be considered to be a lowest-common-denominator implementation only there to provide the most basic defaults.

I can see the appeal of option 3, but IMO anything other than option 1 seems pretty brittle. Presumably with that option, and with the class providing a typealias for Indices, you would no longer have an ambiguity and the code would compile, correct?

Mark

> 
> For reference, Swift 3.0.1 picked DefaultRandomAccessIndices<ReferenceCollection>, current Swift master picks CountableRange<Int>, and my work-in-progress to improve the type checker calls it ambiguous, hence the question :)
> 
> 	- Doug
> 
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