src/jdk.compiler/share/classes/com/sun/tools/javac/code/Preview.java

@@ -225,10 +225,11 @@
       * @return true, if given feature is a preview feature.
       */
      public boolean isPreview(Feature feature) {
          return switch (feature) {
              case PRIMITIVE_PATTERNS -> true;
+             case VALUE_CLASSES -> true;
              //Note: this is a backdoor which allows to optionally treat all features as 'preview' (for testing).
              //When real preview features will be added, this method can be implemented to return 'true'
              //for those selected features, and 'false' for all the others.
              default -> forcePreview;
          };