The tests makes sure that completing java.util. includes 'Set', as in java.util.Set. I'll assume java9 does not move the Set class. I guess there is a change affecting method PackageHelperImpl.getClassnames(), which was originally copied from jline1.
I see it has adaptations for jigsaw made by Cedric: https://github.com/apache/incubator-groovy/commit/0e384ec3 (not pointing fingers, just analyzing the code) And the breaking test follows that new codepath. Debugging a bit, I notice that the files contained in 'jrt:/' does not contains only "/modules/java.base/java/util/Set.class". At a glance, it seems the assumptions in PackageHelperImpl.getPackagesAndClassesFromJigsaw() do not hold with the current Java9 implementation, since it produces a Class java.base.java.util.Set, but not java.util.Set. A quick fix is to change: -if (elems) { - elems = elems[3..<elems.length] +if (elems && elems.length > 2) { + elems = elems[3..<elems.length] in *two* places in that method. I have no idea whether there is a standard for the folder layout FileSystems.newFileSystem(URI.create("jrt:/")) should return, or whether and why this has changed since Cedric made his additions. However this might be related: http://mail.openjdk.java.net/pipermail/jigsaw-dev/2014-November/004044.html
