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

@@ -208,10 +208,11 @@
      public boolean isPreview(Feature feature) {
          return switch (feature) {
              case IMPLICIT_CLASSES -> true;
              case FLEXIBLE_CONSTRUCTORS -> true;
              case PRIMITIVE_PATTERNS -> true;
+             case VALUE_CLASSES -> true;
              case MODULE_IMPORTS -> 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;