Merge pull request #11599 from dotty-staging/fix-9781

Fix #9781: add test

Author: OlivierBlanvillain

tests/pos/i9781.scala

@@ -0,0 +1,19 @@
+trait Txn[T <: Txn[T]]
+
+trait Elem[T <: Txn[T]]
+
+sealed trait State[+T]
+final case class Done[T <: Txn[T]](elem: Elem[T]) extends State[T]
+case object Busy extends State[Nothing]
+
+trait Test[Out <: Txn[Out]] {
+  def apply(opt: Option[State[Out]]): Any = opt match {
+    case Some(state) =>
+      state match {
+        case Done(out)  => "foo"    // problem here
+        case Busy       => throw new IllegalStateException("Cyclic object graph")
+      }
+
+    case None => "bar"
+  }
+}