> On Nov 7, 2016, at 6:16 PM, Douglas Gregor via swift-dev
> <swift-dev@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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