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

[Slava Pestov]

I just tried this and it causes major breakage with associated type inference and the expression checker so its probably more trouble than its worth.

So back to the three options:

1) Do nothing, and give up on fixing https://bugs.swift.org/browse/SR-617 <https://bugs.swift.org/browse/SR-617> for now
2) Finish https://github.com/apple/swift/pull/12174 <https://github.com/apple/swift/pull/12174> and make the new semantics only take effect in -swift-version 5
3) ??? Magic

Slava

> On Oct 6, 2017, at 12:37 AM, Slava Pestov <spestov at apple.com> wrote:
> 
> 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 <mailto: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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>> swift-dev at swift.org <mailto:swift-dev at swift.org>
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> 

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