JEP 443 https://openjdk.org/jeps/443 introduced the "Unnamed Patterns and Variables" preview feature. Consider the following trivial code (also attached as Foo.java):

public class Foo {
   public static void main(final String[] args) throws Exception {
        final String _ = "foo bar";
        System.out.println("Demo done");
   }
}

This uses a unnamed variable.

Now compile it using:

javac -g --enable-preview -source 21 Foo.java

Notice the -g option. The compilation goes through fine. Now try to run that class:

java --enable-preview Foo

This fails with the following error

Error: LinkageError occurred while loading main class Foo
java.lang.ClassFormatError: Illegal field name "" in class Foo

Everything, including running the compiled class, works fine if -g is left out during compilation.

javac version in use:

javac -version
javac 21-ea

java version in use:

java -version
openjdk version "21-ea" 2023-09-19
OpenJDK Runtime Environment (build 21-ea+33-2490)
OpenJDK 64-Bit Server VM (build 21-ea+33-2490, mixed mode, sharing)