Merge pull request #3148 from dotty-staging/fix-2239

Add test case for #2239

Author: liufengyun

tests/pos/i2239.scala

@@ -0,0 +1,14 @@
+trait Rule[-In, +A] extends (In => A) {
+  def flatMap[In2 <: In, B](fa2ruleb: A => Rule[In2, Seq[B]]): Rule[In, Seq[B]] = ???
+  def map[B](fa2b: A => B): Rule[In, B] = ???
+
+  def ~++[In2, B >: A](next: => Rule[In2, Seq[B]]) = for (a <- this; b <- next) yield a :: b.toList
+  // def ~++[In2, B >: A](next: => Rule[In2, Seq[B]]): Rule[In, Seq[B]] = for (a <- this; b <- next) yield a :: b.toList
+}
+
+class SeqRule {
+  type S
+  type A
+  def * : Rule[S, List[A]] = ???
+  def +(rule: Rule[S, A]) : Rule[S, Seq[A]] = rule ~++ *
+}