--add-modules ALL-MODULE-PATH not supported by javadoc tool since Java 13?

[Robert Scholte]

Recently we've started adding Java 13 to our matrix of Java versions when  
testing plugins.
It seems like something has changed, because a set of integration tests  
fails with the following message:

Exit code: 1 - error: --add-modules ALL-MODULE-PATH can only be used when  
compiling the unnamed module

The set belongs to MJAVADOC-568[1], which handles artifacts that get a  
name via the Manifest attribute Automatic-Module-Name.
When creating JavaDoc for such code, using the '--add-modules  
ALL-MODULE-PATH' was the way to get the correct JavaDoc. This used to work  
up to Java 12.

My question: is this an intended change and what's the reasoning behind it?
And of course in the end: what's the preferred solution?

For the integration tests you can have a look at github[3], I can provide  
the results of the ITs when required.

thanks,
Robert

[1] https://issues.apache.org/jira/browse/MJAVADOC-568
[2]  
https://github.com/apache/maven-javadoc-plugin/blob/maven-javadoc-plugin-3.1.1/src/main/java/org/apache/maven/plugins/javadoc/AbstractJavadocMojo.java#L5085-L5090
[3]  
https://github.com/apache/maven-javadoc-plugin/tree/maven-javadoc-plugin-3.1.1/src/it/projects