@UtilityClass should produce a compiler error, when put on a non-static inner class

[tomsit-ionos]

TLDR

When adding @UtilityClass on a non-static inner class with Java 21, then this causes VM crashes and other non-obvious problems.

If u add the static modifier, it works (for me).

See my stripped down reproduction repo @ https://github.com/tomsit-ionos/lombok-utilclass-bug

Also notice that the intellij decompiler is not able to open generated class-file and i verified this also with an online decompiler.

Long story

For me it was a nested class inside a record.

It took me a good day to figure out what's going on b/c i ran into this while upgrade a keycloak extension from KC 25 -> 26.
This set me on the wrong mental path and i was guessing there were some wired upgrade problems at play.

To add to the fun:
The nested class didnt pose a problem for non-testcontainer junit tests using that inner class, ie. these were green!
But only with the intellij junit runner -- which is what I typically use, while working on code, due to the better DX.

What i failed to notice was, that theses normal tests failed when executed by mvn test (e.g. on CI) with this message:

...
[ERROR] The forked VM terminated without properly saying goodbye. VM crash or System.exit called?
...

Originally posted by @tomsit-ionos in #3679 (comment)

[Rawi01]

Interesting, I decompiled the generated class file and the output is different between javac 17 and 21. This difference is also present at the bytecode level.

   ...
   public static final class InnerClass {
      public static final String BAR = "bar";
-     // $FF: synthetic field
-     final OuterClass this$0;

      @Generated
      private InnerClass(OuterClass var1) {
-        this.this$0 = var1;
         throw new UnsupportedOperationException("This is a utility class and cannot be instantiated");
      }
   }

[Rawi01]

TIL: javac does no longer add references to unused outer classes for targets >= Java 18. So this does not cause the problem.

[Rawi01]

I cannot build a class file that is broken, can you reproduce it on another system? Can you also please share the Java version you are using for your build?