Unrelated test for opaque types

Author: odersky

tests/pos/opaques-queue.scala

@@ -0,0 +1,25 @@
+class Elem
+trait QueueSignature:
+  type Queue
+  def empty: Queue
+  def append(q: Queue, e: Elem): Queue
+  def pop(q: Queue): Option[(Elem, Queue)]
+val QueueModule: QueueSignature =
+  object QueueImpl extends QueueSignature:
+    type Queue = (List[Elem], List[Elem])
+    def empty = (Nil, Nil)
+    def append(q: Queue, e: Elem): Queue = (q._1, e :: q._2)
+    def pop(q: Queue): Option[(Elem, Queue)] = q match
+      case (Nil, Nil) => None
+      case (x :: xs, ys) => Some((x, (xs, ys)))
+      case (Nil, ys) => pop((ys.reverse, Nil))
+  QueueImpl
+
+object queues:
+  opaque type Queue = (List[Elem], List[Elem])
+  def empty = (Nil, Nil)
+  def append(q: Queue, e: Elem): Queue = (q._1, e :: q._2)
+  def pop(q: Queue): Option[(Elem, Queue)] = q match
+    case (Nil, Nil) => None
+    case (x :: xs, ys) => Some((x, (xs, ys)))
+    case (Nil, ys) => pop((ys.reverse, Nil))