UPDATE FOR THOSE WHO GOOGLED THIS BUG:
If the project uses source/target 8, adding <source>8</source> in javadoc configuration should make the project buildable on jdk {11, 12, 13}:

  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-javadoc-plugin</artifactId>
    <configuration>
      <source>8</source>
    </configuration>
     ...
  </plugin>

------

Take the simple Maven project with latest plugin dependencies (for example, http://cr.openjdk.java.net/~shade/8212233/javadoc-bug.tar.gz ), and run with JDK 11 and then with current JDK 12 EA. JDK 11 build would succeed (UPD: it would with JDK 11 GA, but not with later updates), and JDK 12 EA would fail on javadoc generation, which looks like a regression. Or maybe the Maven plugins need to do some new incantation to get it right?



I see projects started to disable Javadoc on 12 because of this:
  https://www.google.de/search?q=%22The+code+being+documented+uses+modules+but+the+packages+defined+in%22

$ export JAVA_HOME=~/Install/jdk11-oracle-ga/
$ export PATH=$JAVA_HOME/bin:$PATH
$ java -version
openjdk version "11" 2018-09-25
OpenJDK Runtime Environment 18.9 (build 11+28)
OpenJDK 64-Bit Server VM 18.9 (build 11+28, mixed mode)

$ mvn clean install
...
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2.345 s
[INFO] Finished at: 2018-10-16T11:37:09+02:00
[INFO] Final Memory: 19M/176M
[INFO] ------------------------------------------------------------------------

$ export JAVA_HOME=~/Install/jdk12-oracle-b15/
$ export PATH=$JAVA_HOME/bin:$PATH
$ java -version
openjdk version "12-ea" 2019-03-19
OpenJDK Runtime Environment (build 12-ea+15)
OpenJDK 64-Bit Server VM (build 12-ea+15, mixed mode, sharing)

$ mvn clean install
...
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 4.415 s
[INFO] Finished at: 2018-10-16T11:38:35+02:00
[INFO] Final Memory: 25M/200M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.0.1:jar (attach-javadoc) on project bug: MavenReportException: Error while generating Javadoc:
[ERROR] Exit code: 1 - javadoc: error - The code being documented uses modules but the packages defined in https://docs.oracle.com/javase/8/docs/api/ are in the unnamed module.
[ERROR]