Attempting to implement custom Scala Collection, this code seems to cause the Scala 2.10.2 compiler into an infinite loop :-(

MyCollection.scala

import scala.collection.generic.{GenericCompanion, GenericTraversableTemplate, SeqFactory}
import scala.collection.mutable.{ArrayBuffer, Builder}

class MyCollectionBuilder[A] extends Builder[A, MyCollection[A]] {
  val data = ArrayBuffer[A]()

  def +=(elem: A) = {
    data += elem
    this
  }
  def clear() {
    data.clear()
  }

  def result() = ::(data.head, ::(data.tail.head, ::(data.tail.tail.head, MyCollectionNil))) //TODO recurse! only does 3 at the moment
}

abstract class MyCollection[+A] extends Seq[A]
  with GenericTraversableTemplate[A, MyCollection] {

  override def companion: GenericCompanion[MyCollection] = MyCollection

  def apply(idx: Int) = if(idx == 1) head else tail(idx - 1)

  def length = if(isEmpty) 0 else tail.length + 1

  def iterator = new Iterator[A] {
    var first = true
    var current = this

    def hasNext = {
      current.isEmpty
    }

    def next() = {
        val next = current.tail
        current.head
        current = next
    }
  }
}

object MyCollection extends SeqFactory[MyCollection] {
  def newBuilder[A] = new MyCollectionBuilder[A]()
}


final case class ::[B](val hd: B, val tl: MyCollection[B]) extends MyCollection[B] {
  override def head : B = hd
  override def tail : MyCollection[B] = tl
  override def isEmpty: Boolean = false
}

case object MyCollectionNil extends MyCollection[Nothing] {

}



object Main extends App {
  val c = ::("hello", ::("world", ::("again", MyCollectionNil)))

  c match {
    case head :: tail =>
      println(s"head[$head] tail[$tail]")
  }
}