Updates of strawman

Bring strawman-4 and strawman-5 to feature-parity.
Test also strawman-4.

6 files changed, 867 insertions, 46 deletions

diff --git a/tests/run/colltest4.check b/tests/run/colltest4.check
new file mode 100644
index 000000000..ad3ccb426
--- /dev/null
+++ b/tests/run/colltest4.check
@@ -0,0 +1,86 @@
+Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=256m; support was removed in 8.0
+-------
+123
+123
+1
+1
+Cons(1,Cons(2,Cons(3,Nil)))
+Cons(2,Nil)
+Cons(1,Cons(3,Nil))
+Cons(3,Nil)
+Cons(true,Cons(true,Cons(true,Nil)))
+Cons(1,Cons(-1,Cons(2,Cons(-2,Cons(3,Cons(-3,Nil))))))
+Cons(1,Cons(2,Cons(3,Cons(1,Cons(2,Cons(3,Nil))))))
+Cons(1,Cons(2,Cons(3,Nil)))
+Cons(1,Cons(2,Cons(3,Nil)))
+Cons(1,Cons(2,Cons(3,Cons(a,Nil))))
+Cons((1,true),Cons((2,true),Cons((3,true),Nil)))
+Cons(3,Cons(2,Cons(1,Nil)))
+-------
+123
+123
+1
+1
+Cons(1,Cons(2,Cons(3,Nil)))
+ArrayBuffer(2)
+ArrayBuffer(1, 3)
+ArrayBuffer(3)
+ArrayBuffer(true, true, true)
+ArrayBuffer(1, -1, 2, -2, 3, -3)
+ArrayBuffer(1, 2, 3, 1, 2, 3)
+ArrayBuffer(1, 2, 3)
+Cons(1,Cons(2,Cons(3,Nil)))
+ArrayBuffer(1, 2, 3, a)
+ArrayBuffer((1,true), (2,true), (3,true))
+ArrayBuffer(3, 2, 1)
+-------
+123
+123
+1
+1
+Cons(1,Cons(2,Cons(3,Nil)))
+ListBuffer(2)
+ListBuffer(1, 3)
+ListBuffer(3)
+ListBuffer(true, true, true)
+ListBuffer(1, -1, 2, -2, 3, -3)
+ListBuffer(1, 2, 3, 1, 2, 3)
+ListBuffer(1, 2, 3)
+Cons(1,Cons(2,Cons(3,Nil)))
+ListBuffer(1, 2, 3, a)
+ListBuffer((1,true), (2,true), (3,true))
+ListBuffer(3, 2, 1)
+-------
+123
+123
+1
+1
+Cons(1,Cons(2,Cons(3,Nil)))
+Cons(2,Nil)
+Cons(1,Cons(3,Nil))
+Cons(3,Nil)
+Cons(true,Cons(true,Cons(true,Nil)))
+Cons(1,Cons(-1,Cons(2,Cons(-2,Cons(3,Cons(-3,Nil))))))
+Cons(1,Cons(2,Cons(3,Cons(1,Cons(2,Cons(3,Nil))))))
+Cons(1,Cons(2,Cons(3,Nil)))
+Cons(1,Cons(2,Cons(3,Nil)))
+Cons(1,Cons(2,Cons(3,Cons(a,Nil))))
+Cons((1,true),Cons((2,true),Cons((3,true),Nil)))
+-------
+abc
+abc
+1
+a
+Cons(a,Cons(b,Cons(c,Nil)))
+b
+ac
+c
+Cons(98,Cons(99,Cons(100,Nil)))
+ABC
+a,ab,bc,c
+abcabc
+abcxy
+abc
+Cons(a,Cons(b,Cons(c,Nil)))
+Cons(a,Cons(b,Cons(c,Cons(xyz,Nil))))
+Cons((a,98),Cons((b,99),Cons((c,100),Nil)))
diff --git a/tests/run/colltest4/CollectionStrawMan4_1.scala b/tests/run/colltest4/CollectionStrawMan4_1.scala
new file mode 100644
index 000000000..874f67a2d
--- /dev/null
+++ b/tests/run/colltest4/CollectionStrawMan4_1.scala
@@ -0,0 +1,539 @@
+package strawman.collections
+
+import Predef.{augmentString => _, wrapString => _, _}
+import scala.reflect.ClassTag
+import annotation.unchecked.uncheckedVariance
+import annotation.tailrec
+
+/** A strawman architecture for new collections. It contains some
+ *  example collection classes and methods with the intent to expose
+ *  some key issues. It would be good to compare this to other
+ *  implementations of the same functionality, to get an idea of the
+ *  strengths and weaknesses of different collection architectures.
+ *
+ *  For a test file, see tests/run/CollectionTests.scala.
+ */
+object CollectionStrawMan4 {
+
+  /* ------------ Base Traits -------------------------------- */
+
+  /** Iterator can be used only once */
+  trait IterableOnce[+A] {
+    def iterator: Iterator[A]
+  }
+
+  /** Base trait for instances that can construct a collection from an iterable */
+  trait FromIterable[+C[X] <: Iterable[X]] {
+    def fromIterable[B](v: Iterable[B]): C[B]
+  }
+
+  /** Base trait for companion objects of collections */
+  trait IterableFactory[+C[X] <: Iterable[X]] extends FromIterable[C] {
+    def empty[X]: C[X] = fromIterable(View.Empty)
+    def apply[A](xs: A*): C[A] = fromIterable(View.Elems(xs: _*))
+  }
+
+  /** Base trait for generic collections */
+  trait Iterable[+A] extends IterableOnce[A] with FromIterable[Iterable] {
+    def view: View[A] = View.fromIterator(iterator) // view is overridden, cannot be defined in ops
+    def knownLength: Int = -1
+  }
+
+  /** Base trait for sequence collections */
+  trait Seq[+A] extends Iterable[A] with FromIterable[Seq] {
+    def apply(i: Int): A
+    def length: Int
+  }
+
+  /** Base trait for collection builders */
+  trait Builder[-A, +To] {
+    def +=(x: A): this.type
+    def result: To
+
+    def ++=(xs: IterableOnce[A]): this.type = {
+      xs.iterator.foreach(+=)
+      this
+    }
+  }
+
+  /* ------------ Operations ----------------------------------- */
+
+  /** Operations returning types unrelated to current collection */
+  trait Ops[A] extends Any {
+    def iterator: Iterator[A]
+    def foreach(f: A => Unit): Unit = iterator.foreach(f)
+    def foldLeft[B](z: B)(op: (B, A) => B): B = iterator.foldLeft(z)(op)
+    def foldRight[B](z: B)(op: (A, B) => B): B = iterator.foldRight(z)(op)
+    def indexWhere(p: A => Boolean): Int = iterator.indexWhere(p)
+    def isEmpty: Boolean = !iterator.hasNext
+    def head: A = iterator.next
+  }
+
+  /** Transforms returning same collection type */
+  trait MonoTransforms[A, Repr] extends Any {
+    protected def coll: Iterable[A]
+    protected def fromIterable(it: Iterable[A]): Repr
+    def filter(p: A => Boolean): Repr = fromIterable(View.Filter(coll, p))
+    def partition(p: A => Boolean): (Repr, Repr) = {
+      val pn = View.Partition(coll, p)
+      (fromIterable(pn.left), fromIterable(pn.right))
+    }
+    def drop(n: Int): Repr = fromIterable(View.Drop(coll, n))
+    def to[C[X] <: Iterable[X]](fv: FromIterable[C]): C[A] = fv.fromIterable(coll)
+  }
+
+  trait PolyTransforms[A, C[X]] extends Any {
+    protected def coll: Iterable[A]
+    protected def fromIterable[B](it: Iterable[B]): C[B]
+    def map[B](f: A => B): C[B] = fromIterable(View.Map(coll, f))
+    def flatMap[B](f: A => IterableOnce[B]): C[B] = fromIterable(View.FlatMap(coll, f))
+    def ++[B >: A](xs: IterableOnce[B]): C[B] = fromIterable(View.Concat(coll, xs))
+    def zip[B](xs: IterableOnce[B]): C[(A, B)] = fromIterable(View.Zip(coll, xs))
+  }
+
+  /** Transforms that only apply to Seq */
+  trait MonoTransformsOfSeqs[A, Repr] extends Any with MonoTransforms[A, Repr] {
+    def reverse: Repr = fromIterable(View.Reverse(coll))
+  }
+
+  /** Implementation of Ops for all generic collections */
+  implicit class IterableOps[A](val c: Iterable[A])
+  extends AnyVal with Ops[A] {
+    def iterator = c.iterator
+  }
+
+  /** Implementation of MonoTransforms for all generic collections */
+  implicit class IterableMonoTransforms[A, C[X] <: Iterable[X]](val c: Iterable[A] with FromIterable[C])
+  extends AnyVal with MonoTransforms[A, C[A]] {
+    protected def coll = c
+    protected def fromIterable(it: Iterable[A]): C[A] = c.fromIterable(it)
+  }
+
+  /** Implementation of PolyTransforms for all generic collections */
+  implicit class IterablePolyTransforms[A, C[X] <: Iterable[X]](val c: Iterable[A] with FromIterable[C])
+  extends AnyVal with PolyTransforms[A, C] {
+    protected def coll = c
+    protected def fromIterable[B](it: Iterable[B]): C[B] = c.fromIterable(it)
+  }
+
+  /** Implementation of MonoTransformsForSeqs for all generic collections */
+  implicit class SeqMonoTransforms[A, C[X] <: Seq[X]](val c: Seq[A] with FromIterable[C])
+  extends AnyVal with MonoTransformsOfSeqs[A, C[A]] {
+    protected def coll = c
+    protected def fromIterable(it: Iterable[A]): C[A] = c.fromIterable(it)
+  }
+
+  /* --------- Concrete collection types ------------------------------- */
+
+  /** Concrete collection type: List */
+  sealed trait List[+A] extends Seq[A] with FromIterable[List] { self =>
+    def isEmpty: Boolean
+    def head: A
+    def tail: List[A]
+    def iterator = new Iterator[A] {
+      private[this] var current = self
+      def hasNext = !current.isEmpty
+      def next = { val r = current.head; current = current.tail; r }
+    }
+    def fromIterable[B](c: Iterable[B]): List[B] = List.fromIterable(c)
+    def apply(i: Int): A = {
+      require(!isEmpty)
+      if (i == 0) head else tail.apply(i - 1)
+    }
+    def length: Int =
+      if (isEmpty) 0 else 1 + tail.length
+    def ++:[B >: A](prefix: List[B]): List[B] =
+      if (prefix.isEmpty) this
+      else Cons(prefix.head, prefix.tail ++: this)
+  }
+
+  case class Cons[+A](x: A, private[collections] var next: List[A @uncheckedVariance]) // sound because `next` is used only locally
+  extends List[A] {
+    def isEmpty = false
+    def head = x
+    def tail = next
+  }
+
+  case object Nil extends List[Nothing] {
+    def isEmpty = true
+    def head = ???
+    def tail = ???
+  }
+
+  object List extends IterableFactory[List] {
+    def fromIterator[B](it: Iterator[B]): List[B] =
+      if (it.hasNext) Cons(it.next, fromIterator(it)) else Nil
+    def fromIterable[B](c: Iterable[B]): List[B] = c match {
+      case View.Concat(xs, ys: List[B]) =>
+        fromIterable(xs) ++: ys
+      case View.Drop(xs: List[B], n) =>
+        @tailrec def loop(xs: List[B], n: Int): List[B] =
+          if (n > 0) loop(xs.tail, n - 1) else xs
+        loop(xs, n)
+      case c: List[B] => c
+      case _ => fromIterator(c.iterator)
+    }
+  }
+
+  /** Concrete collection type: ListBuffer */
+  class ListBuffer[A] extends Seq[A] with FromIterable[ListBuffer] with Builder[A, List[A]] {
+    private var first, last: List[A] = Nil
+    private var aliased = false
+    def iterator = first.iterator
+    def fromIterable[B](coll: Iterable[B]) = ListBuffer.fromIterable(coll)
+    def apply(i: Int) = first.apply(i)
+    def length = first.length
+
+    private def copyElems(): Unit = {
+      val buf = ListBuffer.fromIterable(result)
+      first = buf.first
+      last = buf.last
+      aliased = false
+    }
+    def result = {
+      aliased = true
+      first
+    }
+    def +=(elem: A) = {
+      if (aliased) copyElems()
+      val last1 = Cons(elem, Nil)
+      last match {
+        case last: Cons[A] => last.next = last1
+        case _ => first = last1
+      }
+      last = last1
+      this
+    }
+    override def toString: String =
+      if (first.isEmpty) "ListBuffer()"
+      else {
+        val b = new StringBuilder("ListBuffer(").append(first.head)
+        first.tail.foldLeft(b)(_.append(", ").append(_)).append(")").toString
+      }
+  }
+
+  object ListBuffer extends IterableFactory[ListBuffer] {
+    def fromIterable[B](coll: Iterable[B]): ListBuffer[B] = coll match {
+      case pd @ View.Partitioned(partition: View.Partition[B]) =>
+        partition.distribute(new ListBuffer[B]())
+        pd.forced.get.asInstanceOf[ListBuffer[B]]
+      case _ =>
+        new ListBuffer[B] ++= coll
+    }
+  }
+
+  /** Concrete collection type: ArrayBuffer */
+  class ArrayBuffer[A] private (initElems: Array[AnyRef], initLength: Int)
+  extends Seq[A] with FromIterable[ArrayBuffer] with Builder[A, ArrayBuffer[A]] {
+    def this() = this(new Array[AnyRef](16), 0)
+    private var elems: Array[AnyRef] = initElems
+    private var start = 0
+    private var end = initLength
+    def apply(n: Int) = elems(start + n).asInstanceOf[A]
+    def length = end - start
+    override def knownLength = length
+    override def view = new ArrayBufferView(elems, start, end)
+    def iterator = view.iterator
+    def fromIterable[B](it: Iterable[B]): ArrayBuffer[B] =
+      ArrayBuffer.fromIterable(it)
+    def +=(elem: A): this.type = {
+      if (end == elems.length) {
+        if (start > 0) {
+          Array.copy(elems, start, elems, 0, length)
+          end -= start
+          start = 0
+        }
+        else {
+          val newelems = new Array[AnyRef](end * 2)
+          Array.copy(elems, 0, newelems, 0, end)
+          elems = newelems
+        }
+      }
+      elems(end) = elem.asInstanceOf[AnyRef]
+      end += 1
+      this
+    }
+    def result = this
+    def trimStart(n: Int): Unit = start += (n max 0)
+    override def toString = s"ArrayBuffer(${elems.slice(start, end).mkString(", ")})"
+  }
+
+  object ArrayBuffer extends IterableFactory[ArrayBuffer] {
+    def fromIterable[B](c: Iterable[B]): ArrayBuffer[B] = c match {
+      case View.Concat(fst: ArrayBuffer[B], snd: ArrayBuffer[B]) =>
+        val elems = new Array[AnyRef](fst.length + snd.length)
+        Array.copy(fst.elems, fst.start, elems, 0, fst.length)
+        Array.copy(snd.elems, snd.start, elems, fst.length, snd.length)
+        new ArrayBuffer(elems, elems.length)
+      case pd @ View.Partitioned(partition: View.Partition[B]) =>
+        partition.distribute(new ArrayBuffer[B]())
+        pd.forced.get.asInstanceOf[ArrayBuffer[B]]
+      case c if c.knownLength >= 0 =>
+        val elems = new Array[AnyRef](c.knownLength)
+        val it = c.iterator
+        for (i <- 0 until elems.length) elems(i) = it.next().asInstanceOf[AnyRef]
+        new ArrayBuffer[B](elems, elems.length)
+      case _ =>
+        val buf = new ArrayBuffer[B]
+        val it = c.iterator
+        while (it.hasNext) buf += it.next()
+        buf
+    }
+  }
+
+  class ArrayBufferView[A](val elems: Array[AnyRef], val start: Int, val end: Int) extends RandomAccessView[A] {
+    def apply(n: Int) = elems(start + n).asInstanceOf[A]
+  }
+
+  /** Concrete collection type: String */
+  implicit class StringOps(val s: String) extends AnyVal with Ops[Char] {
+    def iterator: Iterator[Char] = new StringView(s).iterator
+  }
+
+  implicit class StringMonoTransforms(val s: String)
+  extends AnyVal with MonoTransformsOfSeqs[Char, String] {
+    protected def coll: Iterable[Char] = StringView(s)
+    protected def fromIterable(it: Iterable[Char]): String = {
+      val sb = new StringBuilder
+      for (ch <- it) sb.append(ch)
+      sb.toString
+    }
+  }
+
+  implicit class StringPolyTransforms(val s: String)
+  extends AnyVal with PolyTransforms[Char, Seq] {
+    protected def coll = StringView(s)
+    protected def fromIterable[B](it: Iterable[B]): Seq[B] = List.fromIterable(it)
+    def map(f: Char => Char): String = {
+      val sb = new StringBuilder
+      for (ch <- s) sb.append(f(ch))
+      sb.toString
+    }
+    def flatMap(f: Char => String) = {
+      val sb = new StringBuilder
+      for (ch <- s) sb.append(f(ch))
+      sb.toString
+    }
+    def ++(xs: IterableOnce[Char]): String = {
+      val sb = new StringBuilder(s)
+      for (ch <- xs.iterator) sb.append(ch)
+      sb.toString
+    }
+    def ++(xs: String): String = s + xs
+  }
+
+  case class StringView(s: String) extends RandomAccessView[Char] {
+    val start = 0
+    val end = s.length
+    def apply(n: Int) = s.charAt(n)
+  }
+
+  /* ------------ Views --------------------------------------- */
+
+  /** A lazy iterable */
+  trait View[+A] extends Iterable[A] with FromIterable[View] {
+    override def view = this
+    override def fromIterable[B](c: Iterable[B]) = c match {
+      case c: View[B] => c
+      case _ => View.fromIterator(c.iterator)
+    }
+  }
+
+  /** Iterator defined in terms of indexing a range */
+  trait RandomAccessView[+A] extends View[A] {
+    def start: Int
+    def end: Int
+    def apply(i: Int): A
+    def iterator: Iterator[A] = new Iterator[A] {
+      private var current = start
+      def hasNext = current < end
+      def next: A = {
+        val r = apply(current)
+        current += 1
+        r
+      }
+    }
+    override def knownLength = end - start max 0
+  }
+
+  object View {
+    def fromIterator[A](it: => Iterator[A]): View[A] = new View[A] {
+      def iterator = it
+    }
+    case object Empty extends View[Nothing] {
+      def iterator = Iterator.empty
+      override def knownLength = 0
+    }
+    case class Elems[A](xs: A*) extends View[A] {
+      def iterator = Iterator(xs: _*)
+      override def knownLength = xs.length
+    }
+    case class Filter[A](val underlying: Iterable[A], p: A => Boolean) extends View[A] {
+      def iterator = underlying.iterator.filter(p)
+    }
+    case class Partition[A](val underlying: Iterable[A], p: A => Boolean) {
+      val left, right = Partitioned(this)
+      // `distribute` makes up for the lack of generic push-based functionality.
+      // It forces both halves of the partition with a given builder.
+      def distribute(bf: => Builder[A, Iterable[A]]) = {
+        val lb, rb = bf
+        val it = underlying.iterator
+        while (it.hasNext) {
+          val x = it.next()
+          (if (p(x)) lb else rb) += x
+        }
+        left.forced = Some(lb.result)
+        right.forced = Some(rb.result)
+      }
+    }
+    case class Partitioned[A](partition: Partition[A]) extends View[A] {
+      private var myForced: Option[Iterable[A]] = None
+      def forced: Option[Iterable[A]] = myForced
+      private[View] def forced_=(x: Option[Iterable[A]]): Unit = myForced = x
+      def underlying = partition.underlying
+      def iterator = forced match {
+        case Some(c) => c.iterator
+        case None =>
+          underlying.iterator.filter(
+            if (this eq partition.left) partition.p else !partition.p(_))
+      }
+    }
+    case class Drop[A](underlying: Iterable[A], n: Int) extends View[A] {
+      def iterator = underlying.iterator.drop(n)
+      override def knownLength =
+        if (underlying.knownLength >= 0) underlying.knownLength - n max 0 else -1
+    }
+    case class Map[A, B](underlying: Iterable[A], f: A => B) extends View[B] {
+      def iterator = underlying.iterator.map(f)
+      override def knownLength = underlying.knownLength
+    }
+    case class FlatMap[A, B](underlying: Iterable[A], f: A => IterableOnce[B]) extends View[B] {
+      def iterator = underlying.iterator.flatMap(f)
+    }
+    case class Concat[A](underlying: Iterable[A], other: IterableOnce[A]) extends View[A] {
+      def iterator = underlying.iterator ++ other
+      override def knownLength = other match {
+        case other: Iterable[_] if underlying.knownLength >= 0 && other.knownLength >= 0 =>
+          underlying.knownLength + other.knownLength
+        case _ =>
+          -1
+      }
+    }
+    case class Zip[A, B](underlying: Iterable[A], other: IterableOnce[B]) extends View[(A, B)] {
+      def iterator = underlying.iterator.zip(other)
+      override def knownLength = other match {
+        case other: Iterable[_] if underlying.knownLength >= 0 && other.knownLength >= 0 =>
+          underlying.knownLength min other.knownLength
+        case _ =>
+          -1
+      }
+    }
+    case class Reverse[A](underlying: Iterable[A]) extends View[A] {
+      def iterator = {
+        var xs: List[A] = Nil
+        val it = underlying.iterator
+        while (it.hasNext) xs = Cons(it.next(), xs)
+        xs.iterator
+      }
+      override def knownLength = underlying.knownLength
+    }
+  }
+
+/* ---------- Iterators ---------------------------------------------------*/
+
+  /** A core Iterator class */
+  trait Iterator[+A] extends IterableOnce[A] { self =>
+    def hasNext: Boolean
+    def next(): A
+    def iterator = this
+    def foldLeft[B](z: B)(op: (B, A) => B): B =
+      if (hasNext) foldLeft(op(z, next))(op) else z
+    def foldRight[B](z: B)(op: (A, B) => B): B =
+      if (hasNext) op(next(), foldRight(z)(op)) else z
+    def foreach(f: A => Unit): Unit =
+      while (hasNext) f(next())
+    def indexWhere(p: A => Boolean): Int = {
+      var i = 0
+      while (hasNext) {
+        if (p(next())) return i
+        i += 1
+      }
+      -1
+    }
+    def filter(p: A => Boolean): Iterator[A] = new Iterator[A] {
+      private var hd: A = _
+      private var hdDefined: Boolean = false
+
+      def hasNext: Boolean = hdDefined || {
+        do {
+          if (!self.hasNext) return false
+          hd = self.next()
+        } while (!p(hd))
+        hdDefined = true
+        true
+      }
+
+      def next() =
+        if (hasNext) {
+          hdDefined = false
+          hd
+        }
+        else Iterator.empty.next()
+    }
+
+    def map[B](f: A => B): Iterator[B] = new Iterator[B] {
+      def hasNext = self.hasNext
+      def next() = f(self.next())
+    }
+
+    def flatMap[B](f: A => IterableOnce[B]): Iterator[B] = new Iterator[B] {
+      private var myCurrent: Iterator[B] = Iterator.empty
+      private def current = {
+        while (!myCurrent.hasNext && self.hasNext)
+          myCurrent = f(self.next()).iterator
+        myCurrent
+      }
+      def hasNext = current.hasNext
+      def next() = current.next()
+    }
+    def ++[B >: A](xs: IterableOnce[B]): Iterator[B] = new Iterator[B] {
+      private var myCurrent: Iterator[B] = self
+      private var first = true
+      private def current = {
+        if (!myCurrent.hasNext && first) {
+          myCurrent = xs.iterator
+          first = false
+        }
+        myCurrent
+      }
+      def hasNext = current.hasNext
+      def next() = current.next()
+    }
+    def drop(n: Int): Iterator[A] = {
+      var i = 0
+      while (i < n && hasNext) {
+        next()
+        i += 1
+      }
+      this
+    }
+    def zip[B](that: IterableOnce[B]): Iterator[(A, B)] = new Iterator[(A, B)] {
+      val thatIterator = that.iterator
+      def hasNext = self.hasNext && thatIterator.hasNext
+      def next() = (self.next(), thatIterator.next())
+    }
+  }
+
+  object Iterator {
+    val empty: Iterator[Nothing] = new Iterator[Nothing] {
+      def hasNext = false
+      def next = throw new NoSuchElementException("next on empty iterator")
+    }
+    def apply[A](xs: A*): Iterator[A] = new RandomAccessView[A] {
+      val start = 0
+      val end = xs.length
+      def apply(n: Int) = xs(n)
+    }.iterator
+  }
+}
+
diff --git a/tests/run/colltest4/CollectionTests_2.scala b/tests/run/colltest4/CollectionTests_2.scala
new file mode 100644
index 000000000..0961180ed
--- /dev/null
+++ b/tests/run/colltest4/CollectionTests_2.scala
@@ -0,0 +1,172 @@
+import Predef.{augmentString => _, wrapString => _, _}
+import scala.reflect.ClassTag
+
+object Test {
+  import strawman.collections._
+  import CollectionStrawMan5._
+
+  def seqOps(xs: Seq[Int]) = {
+    val x1 = xs.foldLeft("")(_ + _)
+    val y1: String = x1
+    val x2 = xs.foldRight("")(_ + _)
+    val y2: String = x2
+    val x3 = xs.indexWhere(_ % 2 == 0)
+    val y3: Int = x3
+    val x4 = xs.head
+    val y4: Int = x4
+    val x5 = xs.to(List)
+    val y5: List[Int] = x5
+    val (xs6, xs7) = xs.partition(_ % 2 == 0)
+    val ys6: Seq[Int] = xs6
+    val ys7: Seq[Int] = xs7
+    val xs8 = xs.drop(2)
+    val ys8: Seq[Int] = xs8
+    val xs9 = xs.map(_ >= 0)
+    val ys9: Seq[Boolean] = xs9
+    val xs10 = xs.flatMap(x => Cons(x, Cons(-x, Nil)))
+    val ys10: Seq[Int] = xs10
+    val xs11 = xs ++ xs
+    val ys11: Seq[Int] = xs11
+    val xs12 = xs ++ Nil
+    val ys12: Seq[Int] = xs12
+    val xs13 = Nil ++ xs
+    val ys13: Seq[Int] = xs13
+    val xs14 = xs ++ Cons("a", Nil)
+    val ys14: Seq[Any] = xs14
+    val xs15 = xs.zip(xs9)
+    val ys15: Seq[(Int, Boolean)] = xs15
+    val xs16 = xs.reverse
+    val ys16: Seq[Int] = xs16
+    println("-------")
+    println(x1)
+    println(x2)
+    println(x3)
+    println(x4)
+    println(x5)
+    println(xs6)
+    println(xs7)
+    println(xs8)
+    println(xs9)
+    println(xs10)
+    println(xs11)
+     println(xs12)
+    println(xs13)
+    println(xs14)
+    println(xs15)
+    println(xs16)
+  }
+
+  def viewOps(xs: View[Int]) = {
+    val x1 = xs.foldLeft("")(_ + _)
+    val y1: String = x1
+    val x2 = xs.foldRight("")(_ + _)
+    val y2: String = x2
+    val x3 = xs.indexWhere(_ % 2 == 0)
+    val y3: Int = x3
+    val x4 = xs.head
+    val y4: Int = x4
+    val x5 = xs.to(List)
+    val y5: List[Int] = x5
+    val (xs6, xs7) = xs.partition(_ % 2 == 0)
+    val ys6: View[Int] = xs6
+    val ys7: View[Int] = xs7
+    val xs8 = xs.drop(2)
+    val ys8: View[Int] = xs8
+    val xs9 = xs.map(_ >= 0)
+    val ys9: View[Boolean] = xs9
+    val xs10 = xs.flatMap(x => Cons(x, Cons(-x, Nil)))
+    val ys10: View[Int] = xs10
+    val xs11 = xs ++ xs
+    val ys11: View[Int] = xs11
+    val xs12 = xs ++ Nil
+    val ys12: View[Int] = xs12
+    val xs13 = Nil ++ xs
+    val ys13: List[Int] = xs13
+    val xs14 = xs ++ Cons("a", Nil)
+    val ys14: View[Any] = xs14
+    val xs15 = xs.zip(xs9)
+    val ys15: View[(Int, Boolean)] = xs15
+    println("-------")
+    println(x1)
+    println(x2)
+    println(x3)
+    println(x4)
+    println(x5)
+    println(xs6.to(List))
+    println(xs7.to(List))
+    println(xs8.to(List))
+    println(xs9.to(List))
+    println(xs10.to(List))
+    println(xs11.to(List))
+    println(xs12.to(List))
+    println(xs13.to(List))
+    println(xs14.to(List))
+    println(xs15.to(List))
+  }
+
+  def stringOps(xs: String) = {
+    val x1 = xs.foldLeft("")(_ + _)
+    val y1: String = x1
+    val x2 = xs.foldRight("")(_ + _)
+    val y2: String = x2
+    val x3 = xs.indexWhere(_ % 2 == 0)
+    val y3: Int = x3
+    val x4 = xs.head
+    val y4: Int = x4
+    val x5 = xs.to(List)
+    val y5: List[Char] = x5
+    val (xs6, xs7) = xs.partition(_ % 2 == 0)
+    val ys6: String = xs6
+    val ys7: String = xs7
+    val xs8 = xs.drop(2)
+    val ys8: String = xs8
+    val xs9 = xs.map(_ + 1)  // !!! need a language change to make this work without the : Char
+    val ys9: Seq[Int] = xs9
+    val xs9a = xs.map(_.toUpper) // !!! need a language change to make this work without the : Char
+    val ys9a: String = xs9a
+    val xs10 = xs.flatMap((x: Char) => s"$x,$x")
+    val ys10: String = xs10
+    val xs11 = xs ++ xs
+    val ys11: String = xs11
+    val xs11a = xs ++ List('x', 'y') // Cons('x', Cons('y', Nil))
+    val ys11a: String = xs11a
+    val xs12 = xs ++ Nil
+    val ys12: String = xs12
+    val xs13 = Nil ++ xs.iterator
+    val ys13: List[Char] = xs13
+    val xs14 = xs ++ Cons("xyz", Nil)
+    val ys14: Seq[Any] = xs14
+    val xs15 = xs.zip(xs9)
+    val ys15: Seq[(Char, Int)] = xs15
+    println("-------")
+    println(x1)
+    println(x2)
+    println(x3)
+    println(x4)
+    println(x5)
+    println(xs6)
+    println(xs7)
+    println(xs8)
+    println(xs9)
+    println(xs9a)
+    println(xs10)
+    println(xs11)
+    println(xs11a)
+    println(xs12)
+    println(xs13)
+    println(xs14)
+    println(xs15)
+  }
+
+  def main(args: Array[String]) = {
+    val ints = Cons(1, Cons(2, Cons(3, Nil)))
+    val intsBuf = ints.to(ArrayBuffer)
+    val intsListBuf = ints.to(ListBuffer)
+    val intsView = ints.view
+    seqOps(ints)
+    seqOps(intsBuf)
+    seqOps(intsListBuf)
+    viewOps(intsView)
+    stringOps("abc")
+  }
+}
diff --git a/tests/run/colltest5.check b/tests/run/colltest5.check
index 6c61d8821..ad3ccb426 100644
--- a/tests/run/colltest5.check
+++ b/tests/run/colltest5.check
@@ -1,3 +1,4 @@
+Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=256m; support was removed in 8.0
 -------
 123
 123
@@ -38,6 +39,23 @@ ArrayBuffer(3, 2, 1)
 1
 1
 Cons(1,Cons(2,Cons(3,Nil)))
+ListBuffer(2)
+ListBuffer(1, 3)
+ListBuffer(3)
+ListBuffer(true, true, true)
+ListBuffer(1, -1, 2, -2, 3, -3)
+ListBuffer(1, 2, 3, 1, 2, 3)
+ListBuffer(1, 2, 3)
+Cons(1,Cons(2,Cons(3,Nil)))
+ListBuffer(1, 2, 3, a)
+ListBuffer((1,true), (2,true), (3,true))
+ListBuffer(3, 2, 1)
+-------
+123
+123
+1
+1
+Cons(1,Cons(2,Cons(3,Nil)))
 Cons(2,Nil)
 Cons(1,Cons(3,Nil))
 Cons(3,Nil)
diff --git a/tests/run/colltest5/CollectionStrawMan5_1.scala b/tests/run/colltest5/CollectionStrawMan5_1.scala
index aa127cb7e..1a89d9659 100644
--- a/tests/run/colltest5/CollectionStrawMan5_1.scala
+++ b/tests/run/colltest5/CollectionStrawMan5_1.scala
@@ -3,6 +3,7 @@ package strawman.collections
 import Predef.{augmentString => _, wrapString => _, _}
 import scala.reflect.ClassTag
 import annotation.unchecked.uncheckedVariance
+import annotation.tailrec
 
 /** A strawman architecture for new collections. It contains some
  *  example collection classes and methods with the intent to expose
@@ -32,30 +33,8 @@ object CollectionStrawMan5 {
     def apply[A](xs: A*): C[A] = fromIterable(View.Elems(xs: _*))
   }
 
-  /** Base trait for Iterable operations */
-  trait IterableLike[+A, +C[X] <: Iterable[X]]
-    extends FromIterable[C]
-       with IterableOps[A]
-       with IterableMonoTransforms[A @uncheckedVariance, C[A @uncheckedVariance]]
-       with IterablePolyTransforms[A @uncheckedVariance, C] {
-    protected def fromLikeIterable(coll: Iterable[A @uncheckedVariance]): C[A @uncheckedVariance] = fromIterable(coll)
-  }
-
-  /** Base trait for Seq operations */
-  trait SeqLike[+A, +C[X] <: Seq[X]] extends IterableLike[A, C] {
-    def reverse: C[A @uncheckedVariance] = {
-      var xs: List[A] = Nil
-      var it = iterator
-      while (it.hasNext) xs = new Cons(it.next, xs)
-      fromLikeIterable(xs)
-    }
-  }
-
   /** Base trait for generic collections */
   trait Iterable[+A] extends IterableOnce[A] with IterableLike[A, Iterable] {
-    override def iterator: Iterator[A]
-    override def fromIterable[B](it: Iterable[B]): Iterable[B]
-
     protected def coll: Iterable[A] = this
     def knownLength: Int = -1
   }
@@ -64,9 +43,9 @@ object CollectionStrawMan5 {
   trait Seq[+A] extends Iterable[A] with SeqLike[A, Seq] {
     def apply(i: Int): A
     def length: Int
-    override def iterator: Iterator[A]
   }
 
+  /** Base trait for strict collections */
   trait Buildable[+A, +To <: Iterable[A]] extends Iterable[A] {
     protected[this] def newBuilder: Builder[A, To]
     override def partition(p: A => Boolean): (To, To) = {
@@ -74,8 +53,11 @@ object CollectionStrawMan5 {
       iterator.foreach(x => (if (p(x)) l else r) += x)
       (l.result, r.result)
     }
+    // one might also override other transforms here to avoid generating
+    // iterators if it helps efficiency.
   }
 
+  /** Base trait for collection builders */
   trait Builder[-A, +To] {
     def +=(x: A): this.type
     def result: To
@@ -88,6 +70,29 @@ object CollectionStrawMan5 {
 
   /* ------------ Operations ----------------------------------- */
 
+  /** Base trait for Iterable operations
+   *
+   *  VarianceNote
+   *  ============
+   *
+   *  We require that for all child classes of Iterable the variance of
+   *  the child class and the variance of the `C` parameter passed to `IterableLike`
+   *  are the same. We cannot express this since we lack variance polymorphism. That's
+   *  why we have to resort at some places to write `C[A @uncheckedVariance]`.
+   *
+   */
+  trait IterableLike[+A, +C[X] <: Iterable[X]]
+    extends FromIterable[C]
+       with IterableOps[A]
+       with IterableMonoTransforms[A, C[A @uncheckedVariance]] // sound bcs of VarianceNote
+       with IterablePolyTransforms[A, C] {
+    protected[this] def fromLikeIterable(coll: Iterable[A]): C[A] = fromIterable(coll)
+  }
+
+  /** Base trait for Seq operations */
+  trait SeqLike[+A, +C[X] <: Seq[X]]
+  extends IterableLike[A, C] with SeqMonoTransforms[A, C[A @uncheckedVariance]] // sound bcs of VarianceNote
+
   trait IterableOps[+A] extends Any {
     def iterator: Iterator[A]
     def foreach(f: A => Unit): Unit = iterator.foreach(f)
@@ -99,16 +104,19 @@ object CollectionStrawMan5 {
     def view: View[A] = View.fromIterator(iterator)
   }
 
-  trait IterableMonoTransforms[A, +Repr] extends Any {
+  trait IterableMonoTransforms[+A, +Repr] extends Any {
     protected def coll: Iterable[A]
-    protected def fromLikeIterable(coll: Iterable[A]): Repr
+    protected[this] def fromLikeIterable(coll: Iterable[A]): Repr
     def filter(p: A => Boolean): Repr = fromLikeIterable(View.Filter(coll, p))
     def partition(p: A => Boolean): (Repr, Repr) = {
       val pn = View.Partition(coll, p)
       (fromLikeIterable(pn.left), fromLikeIterable(pn.right))
     }
     def drop(n: Int): Repr = fromLikeIterable(View.Drop(coll, n))
-    def to[C[X] <: Iterable[X]](fi: FromIterable[C]): C[A] = fi.fromIterable(coll)
+    def to[C[X] <: Iterable[X]](fi: FromIterable[C]): C[A @uncheckedVariance] =
+      // variance seems sound because `to` could just as well have been added
+      // as a decorator. We should investigate this further to be sure.
+      fi.fromIterable(coll)
   }
 
   trait IterablePolyTransforms[+A, +C[A]] extends Any {
@@ -118,9 +126,10 @@ object CollectionStrawMan5 {
     def flatMap[B](f: A => IterableOnce[B]): C[B] = fromIterable(View.FlatMap(coll, f))
     def ++[B >: A](xs: IterableOnce[B]): C[B] = fromIterable(View.Concat(coll, xs))
     def zip[B](xs: IterableOnce[B]): C[(A @uncheckedVariance, B)] = fromIterable(View.Zip(coll, xs))
+       // sound bcs of VarianceNote
   }
 
-  trait SeqMonoTransforms[A, +Repr] extends Any with IterableMonoTransforms[A, Repr] {
+  trait SeqMonoTransforms[+A, +Repr] extends Any with IterableMonoTransforms[A, Repr] {
     def reverse: Repr = {
       var xs: List[A] = Nil
       var it = coll.iterator
@@ -156,10 +165,12 @@ object CollectionStrawMan5 {
       case xs: List[B] => this ++: xs
       case _ => super.++(xs)
     }
-    override def reverse = super.reverse
+    @tailrec final override def drop(n: Int) =
+      if (n > 0) tail.drop(n - 1) else this
   }
 
-  case class Cons[+A](x: A, private[collections] var next: List[A @uncheckedVariance]) extends List[A] {
+  case class Cons[+A](x: A, private[collections] var next: List[A @uncheckedVariance]) // sound because `next` is used only locally
+  extends List[A] {
     override def isEmpty = false
     override def head = x
     def tail = next
@@ -182,11 +193,7 @@ object CollectionStrawMan5 {
   class ListBuffer[A] extends Seq[A] with SeqLike[A, ListBuffer] with Builder[A, List[A]] {
     private var first, last: List[A] = Nil
     private var aliased = false
-    def iterator = new Iterator[A] {
-      var current: List[A] = first
-      def hasNext = ???
-      def next = ???
-    }
+    def iterator = first.iterator
     def fromIterable[B](coll: Iterable[B]) = ListBuffer.fromIterable(coll)
     def apply(i: Int) = first.apply(i)
     def length = first.length
@@ -211,6 +218,12 @@ object CollectionStrawMan5 {
       last = last1
       this
     }
+    override def toString: String =
+      if (first.isEmpty) "ListBuffer()"
+      else {
+        val b = new StringBuilder("ListBuffer(").append(first.head)
+        first.tail.foldLeft(b)(_.append(", ").append(_)).append(")").toString
+      }
   }
 
   object ListBuffer extends IterableFactory[ListBuffer] {
@@ -297,12 +310,12 @@ object CollectionStrawMan5 {
     def fromIterable[B](coll: Iterable[B]): List[B] = List.fromIterable(coll)
     def map(f: Char => Char): String = {
       val sb = new StringBuilder
-      for (ch <- StringOps(s)) sb.append(f(ch))
+      for (ch <- s) sb.append(f(ch))
       sb.toString
     }
     def flatMap(f: Char => String): String = {
       val sb = new StringBuilder
-      for (ch <- StringOps(s)) sb.append(f(ch))
+      for (ch <- s) sb.append(f(ch))
       sb.toString
     }
     def ++(xs: IterableOnce[Char]): String = {
@@ -399,15 +412,6 @@ object CollectionStrawMan5 {
           -1
       }
     }
-    case class Reverse[A](underlying: Iterable[A]) extends View[A] {
-      def iterator = {
-        var xs: List[A] = Nil
-        val it = underlying.iterator
-        while (it.hasNext) xs = Cons(it.next(), xs)
-        xs.iterator
-      }
-      override def knownLength = underlying.knownLength
-    }
   }
 
 /* ---------- Iterators ---------------------------------------------------*/
diff --git a/tests/run/colltest5/CollectionTests_2.scala b/tests/run/colltest5/CollectionTests_2.scala
index cdff2e260..0961180ed 100644
--- a/tests/run/colltest5/CollectionTests_2.scala
+++ b/tests/run/colltest5/CollectionTests_2.scala
@@ -124,7 +124,7 @@ object Test {
     val ys9: Seq[Int] = xs9
     val xs9a = xs.map(_.toUpper) // !!! need a language change to make this work without the : Char
     val ys9a: String = xs9a
-    val xs10 = xs.flatMap(x => s"$x,$x")
+    val xs10 = xs.flatMap((x: Char) => s"$x,$x")
     val ys10: String = xs10
     val xs11 = xs ++ xs
     val ys11: String = xs11
@@ -161,9 +161,11 @@ object Test {
   def main(args: Array[String]) = {
     val ints = Cons(1, Cons(2, Cons(3, Nil)))
     val intsBuf = ints.to(ArrayBuffer)
+    val intsListBuf = ints.to(ListBuffer)
     val intsView = ints.view
     seqOps(ints)
     seqOps(intsBuf)
+    seqOps(intsListBuf)
     viewOps(intsView)
     stringOps("abc")
   }