Fix #9841: add test

Author: liufengyun

tests/patmat/i9841.scala

@@ -0,0 +1,19 @@
+trait Txn[T <: Txn[T]]
+
+object Impl {
+  sealed trait Entry[T <: Txn[T], A]
+  case class EntrySingle[T <: Txn[T], A](term: Long, v: A)  extends Entry[T, A]
+}
+
+trait Impl[T <: Txn[T], K] {
+  import Impl._
+
+  def put[A](): Unit = {
+    val opt: Option[Entry[T, A]] = ???
+    
+    opt match {
+      case Some(EntrySingle(_, prevValue)) => ???   // crashes
+      case _ =>
+    }
+  }
+}