Add another test

Author: liufengyun

tests/pos/i9841b.scala

@@ -0,0 +1,25 @@
+trait Exec[T <: Exec[T]]
+
+object Tree {
+  sealed trait Next[+T, +PL, +P, +H, +A]
+
+  sealed trait Child[+T, +PL, +P, +H, +A]
+
+  sealed trait Branch[T <: Exec[T], PL, P, H, A] extends Child[T, PL, P, H, A] with NonEmpty[T, PL, P, H]
+
+  sealed trait NonEmpty[T <: Exec[T], PL, P, H]
+
+  case object Empty extends Next[Nothing, Nothing, Nothing, Nothing, Nothing]
+
+  sealed trait RightBranch[T <: Exec[T], PL, P, H, A] extends Next[T, PL, P, H, A] with Branch[T, PL, P, H, A] 
+
+  trait BranchImpl[T <: Exec[T], PL, P, H, A] {
+    def next: Next[T, PL, P, H, A]
+
+    def nextOption: Option[Branch[T, PL, P, H, A]] =
+      next match {  // crashes
+        case b: RightBranch[T, PL, P, H, A] => Some(b)
+        case Empty                          => None
+      }
+  }
+}