Add test case for higher-kinded operations on collections

The test case that caused all previous commits in this branch.

Author: odersky

tests/pos/Iterable.scala

@@ -0,0 +1,52 @@
+package dotty.collections
+package immutable
+
+import annotation.unchecked.uncheckedVariance
+
+trait Collection[+CC[X] <: Collection[CC, X], T] {
+  def companion: CollectionCompanion[CC]
+}
+
+trait Iterable[T] extends Collection[Iterable, T] {
+  def iterator: Iterator[T]
+  override def companion: IterableCompanion[Iterable] = Iterable
+}
+
+trait Seq[T] extends Iterable[T] with Collection[Seq, T] {
+  def apply(x: Int): T
+  override def companion: IterableCompanion[Seq] = Seq
+}
+
+abstract class CollectionCompanion[+CC[X] <: Collection[CC, X]]
+
+trait IterableImpls[CC[X]] {
+  def fromIterator[T](it: Iterator[T]): CC[T]
+  def toIterator[T](xs: CC[T]): Iterator[T]
+  def map[T, U](xs: CC[T], f: T => U): CC[U] =
+    fromIterator(toIterator(xs).map(f))
+  def filter[T](xs: CC[T], p: T => Boolean): CC[T] =
+    fromIterator(toIterator(xs).filter(p))
+  def flatMap[T, U](xs: CC[T], f: T => TraversableOnce[U]): CC[U] =
+    fromIterator(toIterator(xs).flatMap(f))
+}
+
+abstract class IterableCompanion[+CC[X] <: Iterable[X] with Collection[CC, X]]
+extends CollectionCompanion[CC] with IterableImpls[CC] @uncheckedVariance {
+  def toIterator[T](xs: CC[T] @uncheckedVariance) = xs.iterator
+  implicit def transformOps[T](xs: CC[T] @uncheckedVariance): TransformOps[CC, T] = new TransformOps[CC, T](xs)
+}
+
+class TransformOps[+CC[X] <: Iterable[X] with Collection[CC, X], T] (val xs: CC[T]) extends AnyVal {
+  def companion[T](xs: CC[T] @uncheckedVariance): IterableCompanion[CC] = xs.companion.asInstanceOf
+  def map[U](f: T => U): CC[U] = companion(xs).map(xs, f)
+  def filter(p: T => Boolean): CC[T] = companion(xs).filter(xs, p)
+  def flatMap[U](f: T => TraversableOnce[U]): CC[U] = companion(xs).flatMap(xs, f)
+}
+
+object Iterable extends IterableCompanion[Iterable] {
+  def fromIterator[T](it: Iterator[T]): Iterable[T] = ???
+}
+object Seq extends IterableCompanion[Seq] {
+  def fromIterator[T](it: Iterator[T]): Seq[T] = ???
+}
+