Merge pull request #12542 from dotty-staging/fix-11973

Fix #11973: Add regression test

Author: bishabosha

tests/pos/11973.scala

@@ -0,0 +1,9 @@
+enum E:
+  case C
+
+trait T
+
+def f(x: E | T): Unit = x match {
+  case e: E => ()
+  case t: T => ()
+}