Swift 3 Collection protocol bug on Xcode 8 beta

BugSample.swift

// Copy the following code into a playground on Xcode 8 beta
import Foundation

class CustomCollection: Collection {
  typealias Index = Int
  typealias IndexDistance = Int
  
  private(set) var count: Int
  
  private var cursor = 0
  func next() -> Int? {
    defer {
      cursor += 1
    }
    if cursor < self.count {
      return cursor
    }
    return nil
  }
  
  subscript(_ idx: Index) -> Int {
    if idx < count {
      return idx
    }
    return 0
  }
  
  var startIndex: Index {
    return 0
  }
  
  var endIndex: Index {
    return count
  }
  
  func index(after i: Index) -> Index {
    return i + 1
  }
  
  init(count: Int) {
    self.count = count
  }
}

// This class cannot use for loop on its instances
class DerivedCustomCollection : CustomCollection {
}

// This class cannot use for loop on instance.enumerated()
class DerivedCustomCollectionWithCustomIterator : CustomCollection {
  typealias Iterator = IndexingIterator<DerivedCustomCollectionWithCustomIterator>
}

let collection1 = DerivedCustomCollection(count: 5)
let collection2 = DerivedCustomCollectionWithCustomIterator(count: 5)

// Swift Compiler Error: 
// 'IndexingIterator<DerivedCustomCollection>' is not convertible to 'Iterator' (aka 'IndexingIterator<CustomCollection>')
for i in collection1 {
  print(i)
}

// OK
for (o, i) in collection1.enumerated() {
  print("\(o) \(i)")
}

// OK
for i in collection2 {
  print(i)
}

// Swift Compiler Error:
// Expression type 'EnumeratedSequence<DerivedCustomCollectionWithCustomIterator>' is ambiguous without more context
for (o, i) in collection2.enumerated() {
  print("\(o) \(i)")
}