[swift-dev] Should non-final classes be allowed to conform to Collection?

[Slava Pestov]

Another solution is to change the Collection protocol as follows,

protocol Collection {
  associatedtype ConformingType = Self
  associatedtype Iterator = IndexingIterator<ConformingType>

  …
}

extension Collection where Iterator == IndexingIterator<ConformingType> {
  func makeIterator() -> IndexingIterator<ConformingType> { … }
}

I believe this will fix the source compatibility issue and also make ‘for x in Derived()’ type check. The downside is that the witness table for a Collection conformance now stores an additional associated type for the static conforming class type. However that’s exactly what you need to store somewhere to make this work for non-final classes.

Slava

> On Oct 6, 2017, at 12:25 AM, Slava Pestov via swift-dev <swift-dev at swift.org> wrote:
> 
> Hi all,
> 
> Consider this code,
> 
> class Base : Collection {
>   var startIndex: Int { return 0 }
> 
>   var endIndex: Int { return 10 }
> 
>   func index(after i: Int) -> Int { return i + 1 }
> 
>   subscript(index: Int) -> Int { return index }
> }
> 
> We infer the associated type ‘Iterator’ as ‘IndexingIterator<Base>’. I can use an instance of Base as a sequence just fine:
> 
> for x in Base() {} // OK
> 
> Now if I subclass Base, the associated type is still ‘IndexingIterator<Base>’:
> 
> class Derived : Base {}
> 
> However the implementation of makeIterator is defined in a constrained extension by the standard library,
> 
> extension Collection where Self.Iterator == IndexingIterator<Self> {
>   func makeIterator() -> IndexingIterator<Self> { … }
> }
> 
> So I cannot call it on a subclass:
> 
> for x in Derived() {} // fails
> 
> The error is bizarre, "'IndexingIterator<Base>' is not convertible to 'IndexingIterator<Derived>’” — I’m not doing a conversion here.
> 
> If you try to call makeIterator() directly, you get an ambiguity error instead:
> 
> col.swift:17:5: error: ambiguous reference to member 'makeIterator()'
> _ = Derived().makeIterator()
>     ^~~~~~~~~
> Swift.Collection:6:17: note: found this candidate
>     public func makeIterator() -> IndexingIterator<Self>
>                 ^
> Swift.Sequence:5:17: note: found this candidate
>     public func makeIterator() -> Self
>                 ^
> 
> Now I couldn’t come up with an example where the code compiles but crashes at runtime because of a type mismatch, but it’s not outside the realm of possibility.
> 
> With my PR here the conformance itself no longer type checks: https://github.com/apple/swift/pull/12174 <https://github.com/apple/swift/pull/12174>
> 
> col.swift:1:7: error: type 'Base' does not conform to protocol 'Collection'
> class Base : Collection {
>       ^
> Swift.Sequence:5:17: note: candidate has non-matching type '<Self> () -> Self' [with Element = Int, Index = Int, IndexDistance = Int, Iterator = IndexingIterator<Base>, SubSequence = Slice<Base>, Indices = DefaultIndices<Base>]
>     public func makeIterator() -> Self
>                 ^
> Swift.Collection:6:17: note: candidate has non-matching type '<Self> () -> IndexingIterator<Self>' [with Element = Int, Index = Int, IndexDistance = Int, Iterator = IndexingIterator<Base>, SubSequence = Slice<Base>, Indices = DefaultIndices<Base>]
>     public func makeIterator() -> IndexingIterator<Self>
> 
> I found one example in our code base where this pattern comes up, and that’s SyntaxCollection in tools/SwiftSyntax/SyntaxCollection.swift. It has no subclasses so making it final works there.
> 
> This was reported externally as https://bugs.swift.org/browse/SR-1863 <https://bugs.swift.org/browse/SR-1863>. I’m not sure if the user expects it to work or just to produce a reasonable diagnostic instructing them to make the class final.
> 
> What does everyone think of this?
> 
> 1) Can anyone suggest a way to make it work, so that ‘for x in Derived()’ type checks and the correct Self type (Base, not Derived) for the substitution?
> 
> 2) Should we just ban such ’non-covariant’ conformances? There is precedent for this — in Swift 3, we used to allow non-final classes to conform to protocols whose requirements had same-type constraints with the right hand side equal to ‘Self’, and Doug closed this hole in Swift 4. My PR is essentially a more comprehensive fix for this hole.
> 
> 3) Should we allow the hole to remain in place, admitting non-final classes that model Collection, at the cost of not being able to ever fix https://bugs.swift.org/browse/SR-617 <https://bugs.swift.org/browse/SR-617>?
> 
> Slava
> 
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