Since JDK-8240169 javadoc is able to create external links to libraries documented with "mismatching" modularity. However, a warning such as the following is generated when this happens:

warning: The code being documented uses packages in the unnamed module, but the packages defined in <URL> are in named modules.

This is a problem for projects using the -Werror option to treat warnings as errors. The message should probably be purely informational and not a warning.

Update: It was decided that the warning should remain as default behaviour, but it should be possible to disable it via a new option.