FULL PRODUCT VERSION :
java version "9-ea"
Java(TM) SE Runtime Environment (build 9-ea+154)
Java HotSpot(TM) 64-Bit Server VM (build 9-ea+154, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Linux xxx 4.4.0-59-generic #80-Ubuntu SMP Fri Jan 6 17:47:47 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
When generating Javadoc for JOSM with Java9-EA build 154 we face the attached error.

Please add the label "josm-found" as per Quality Outreach.

REGRESSION. Last worked in version 8u121

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
svn co https://josm.openstreetmap.de/svn/trunk/
ant javadoc

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
no error

ERROR MESSAGES/STACK TRACES THAT OCCUR :
[javadoc] Standard Doclet version 9-ea
  [javadoc] Building tree for all the packages and classes...
  [javadoc] /var/lib/jenkins/jobs/Java-EarlyAccess-JOSM/workspace/jdk/JDK9/src/org/openstreetmap/josm/data/validation/routines/package.html:295: error: header used out of sequence: <H3>
  [javadoc] Generating src-html/org/openstreetmap/josm/tools/template_engine/Variable.html...
  [javadoc] 2 errors
  [javadoc] 22 warnings
  [javadoc] javadoc: error - An internal exception has occurred.
  [javadoc] (java.lang.UnsupportedOperationException: should not happen)
  [javadoc] Please file a bug against the javadoc tool via the Java bug reporting page
  [javadoc] (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com)
  [javadoc] for duplicates. Include error messages and the following diagnostic in your report. Thank you.
  [javadoc] java.lang.UnsupportedOperationException: should not happen
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.Utils$13.defaultAction(Utils.java:1896)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.Utils$13.defaultAction(Utils.java:1869)
  [javadoc] at java.compiler/javax.lang.model.util.SimpleTypeVisitor6.visitError(SimpleTypeVisitor6.java:180)
  [javadoc] at jdk.compiler/com.sun.tools.javac.code.Type$ErrorType.accept(Type.java:2394)
  [javadoc] at java.compiler/javax.lang.model.util.AbstractTypeVisitor6.visit(AbstractTypeVisitor6.java:104)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.Utils.getQualifiedTypeName(Utils.java:1898)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.Utils.lambda$makeTypeMirrorClassUseComparator$4(Utils.java:1840)
  [javadoc] at java.base/java.util.TreeMap.compare(TreeMap.java:1292)
  [javadoc] at java.base/java.util.TreeMap.put(TreeMap.java:536)
  [javadoc] at java.base/java.util.TreeSet.add(TreeSet.java:255)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.ClassUseMapper.mapExecutable(ClassUseMapper.java:348)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.ClassUseMapper.<init>(ClassUseMapper.java:246)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.ClassUseWriter.generate(ClassUseWriter.java:181)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.HtmlDoclet.generateOtherFiles(HtmlDoclet.java:131)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.AbstractDoclet.startGeneration(AbstractDoclet.java:213)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.AbstractDoclet.run(AbstractDoclet.java:117)
  [javadoc] at jdk.javadoc/jdk.javadoc.doclets.StandardDoclet.run(StandardDoclet.java:71)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.tool.Start.parseAndExecute(Start.java:604)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:424)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:341)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.tool.Main.execute(Main.java:63)
  [javadoc] at jdk.javadoc/jdk.javadoc.internal.tool.Main.main(Main.java:52)

REPRODUCIBILITY :
This bug can be reproduced always.