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

[Slava Pestov]

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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