public struct DefaultIndex<Base: Sequence> : Comparable {
  // If the next line is commented out, there's no compiler crash
  var element: Base.Element? = nil
  
  init() {}

  public static func ==(lhs: DefaultIndex, rhs: DefaultIndex) -> Bool {
    return true
  }
  public static func < (lhs: DefaultIndex, rhs: DefaultIndex) -> Bool {
    return false
  }
}

extension Collection where Self.Index == DefaultIndex<Self> {
  public var startIndex: Index {
    return DefaultIndex()
  }
  
  public var endIndex: Index {
    return DefaultIndex()
  }
  
  public subscript(i: Index) -> Iterator.Element {
    fatalError()
  }
  
  public func index(after i: Index) -> Index {
    return i
  }
}

// If the following line is commented out, there's no compiler crash
struct ThisIsFine: Collection {}