add regression test for pattern match on path-dependent type (#17001)

Author: dwijnand

tests/pos/t16827.scala

@@ -0,0 +1,9 @@
+// scalac: -Werror
+
+trait Outer[F[_]]:
+  sealed trait Inner
+  trait Inner1 extends Inner
+  def foo(rv: Either[Inner, Int]) =
+    rv match
+      case Right(_) =>
+      case Left(_) =>