diff -r 5c0e23d73cea -r 9efdebe24c65 src/Pure/library.scala
--- a/src/Pure/library.scala	Mon Mar 01 20:04:33 2021 +0100
+++ b/src/Pure/library.scala	Mon Mar 01 20:12:09 2021 +0100
@@ -285,6 +285,7 @@
 
   def is_subclass[A, B](a: Class[A], b: Class[B]): Boolean =
   {
+    import scala.language.existentials
     @tailrec def subclass(c: Class[_]): Boolean =
     {
       c == b ||