Maven compiler plugin doesn't support mixing module-path and class-path. Just remove module-info.java file and it will revert to classpath.

Because supporting Java 9 is not the same as turning whole project into Java 9 modules? The idea around J9 is that you should be able to use existing non-modular projects by simply putting them on classpath and it would work as it used to before.

Turning the whole project into Java 9 modules is not the topic of this issue.
My point is "I understand we can discuss about the interest of making Weld modular (by providing explicit module-info.java files) but I don't understand that we decide to close the discussion before it begins because of the location 3 classes".

FYI I opened a similar issue to rest-easy (RESTEASY-1480) that was solved in a few days.