Fix #12279: Add test

Author: liufengyun

tests/patmat/i12279.scala

@@ -0,0 +1,9 @@
+import scala.reflect.Typeable
+
+def unionTypeTest[T: Typeable](m: Int|T) =
+  m match
+  case x: Int => println("Got Int")
+  case t: T => println("Got T")
+
+@main def run =
+  unionTypeTest(())