[BUG] @UtilityClass fails to make inner class static in Java 9+

[liblit]

Describe the bug

When a class is annotated with @UtilityClass, and this class has an inner class, the inner class is also treated as static in Java 1.8, but is not treated as static in Java 9 and later. This causes compilation failure under Java 9+ for code that compiled correctly in Java 1.8.

To Reproduce

Save the following as Outer.java:

import lombok.experimental.UtilityClass;

@UtilityClass
class Outer {

  Object buildInner() {
    return new Inner();
  }

  class Inner {}
}

Compile this file using javac from Java 1.8, and observe that compilation succeeds with no diagnostic output:

% /path/to/java-1.8/bin/javac -cp lombok.jar Outer.java

Now compile this file using javac from Java 9 or later, and observe that compilation fails:

% /path/to/java-9/bin/javac -cp lombok.jar Outer.java
Outer.java:7: error: non-static variable this cannot be referenced from a static context
    return new Inner();
           ^
1 error

Expected behavior

Java 9 and later compilers should have compiled this source file, producing no diagnostic output.

Version info (please complete the following information):

• Lombok version: 1.18.22
• Platform: javac 9

Additional context

The problem first arises with Java 9, but also occurs in Java 11, 14, and 17. (I haven't tested Java 10, 12, 13, 15, or 16.)

#2519 seems closely related, but was closed because it could not be reproduced. However, that earlier report mentioned nothing about different Java versions. In this new report, I find that the Java compiler version is critical: the problem only arises in Java 9 and later, not in Java 1.8.

[rzwitserloot]

Heh, yeah that's no doubt what happened with #2519. Nice find. No need to test on the non-LTS versions except the last for such things.

[Rawi01]

The generated code is fine but we also have to adjust the related compiler symbols, in this case we have to remove the enclosing type of the inner class.

[TysonMN]

I think the first release containing this fix is version 1.18.24.

[xinbimingjingtai]

I need this commit to make lombok.accessors.capitalization work in IDE