Open Radar

 
According to the CollectionType documentation:

    /// - Note: this associated type appears as a requirement in
    ///   `SequenceType`, but is restated here with stricter
    ///   constraints: in a `CollectionType`, the `SubSequence` should
    ///   also be a `CollectionType`.
    typealias SubSequence : Indexable, SequenceType = Slice<Self>

However, CollectionType.SubSequence is not a CollectionType. It is only an Indexable SequenceType. Moreover, SubSequence is not a requirement in SequenceType.

This means the following code, which I would expect to compile, fails because Seq.SubSequence is not guaranteed to be a CollectionType:

func count<Seq: CollectionType>(xs: Seq) -> Int {
    guard !xs.isEmpty else { return 0 }
    return 1 + count(dropFirst(xs)) // Cannot find an overload for ‘count’ that accepts an argument list of type ‘(Seq.SubSequence)’
}