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.
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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