If you take jsr166 CVS of 2017-02-18 and apply the error-provoking patch below:

--- build.xml 18 Feb 2017 18:33:42 -0000 1.234
+++ build.xml 18 Feb 2017 18:42:02 -0000
@@ -456,7 +456,6 @@
       <arg line="-Xdocrootparent ${java9.docroot.url}"/>
       <arg line="-Xmaxerrs 1000 -Xmaxwarns 1000"/>
       <arg value="-XDignore.symbol.file=true"/>
- <arg value="--patch-module=java.base=${src.dir}"/>
       <arg value="-tag"/>
       <arg value="${javadoc.jls.option}"/>
 <!-- @apiNote currently unused -->

then "ant -v -emacs docs" fails thus:

Executing '/home/martin/jdk/jdk9/bin/javadoc' with arguments:
'-d'
'/home/martin/jsr166/javadoc/build/docs'
'-overview'
'/home/martin/jsr166/javadoc/src/main/intro.html'
'-protected'
'-Xdocrootparent'
'http://download.java.net/java/jdk9/docs&#39;
'-Xmaxerrs'
'1000'
'-Xmaxwarns'
'1000'
'-XDignore.symbol.file=true'
'-tag'
'jls:a:See <cite>The Java&trade; Language Specification</cite>:'
'-tag'
'implSpec:a:Implementation Requirements:'
'-tag'
'implNote:a:Implementation Note:'
'-version'
'-author'
'-sourcepath'
'/home/martin/jsr166/javadoc/src/main'
'-link'
'http://download.java.net/java/jdk9/docs/api/&#39;
'/home/martin/jsr166/javadoc/src/main/java/util/AbstractQueue.java'
...

Generating /home/martin/jsr166/javadoc/build/docs/java/util/package-tree.html...
102 errors
280 warnings
javadoc: error - An internal exception has occurred.
(java.lang.NullPointerException)
Please file a bug against the javadoc tool via the Java bug reporting page
(http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com)
for duplicates. Include error messages and the following diagnostic in your report. Thank you.
java.lang.NullPointerException
at jdk.compiler/com.sun.tools.javac.code.Types.erasure(Types.java:2090)
at jdk.compiler/com.sun.tools.javac.code.Types.access$800(Types.java:83)
at jdk.compiler/com.sun.tools.javac.code.Types$14.visitTypeVar(Types.java:2148)
at jdk.compiler/com.sun.tools.javac.code.Types$14.visitTypeVar(Types.java:2098)
at jdk.compiler/com.sun.tools.javac.code.Type$TypeVar.accept(Type.java:1661)
at jdk.compiler/com.sun.tools.javac.code.Types$DefaultTypeVisitor.visit(Types.java:4613)
at jdk.compiler/com.sun.tools.javac.code.Types.erasure(Types.java:2093)
at jdk.compiler/com.sun.tools.javac.code.Types.erasure(Types.java:2079)
at jdk.compiler/com.sun.tools.javac.model.JavacTypes.erasure(JavacTypes.java:133)
at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.Utils$5.visitTypeVariable(Utils.java:1202)
at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.Utils$5.visitTypeVariable(Utils.java:1180)
at jdk.compiler/com.sun.tools.javac.code.Type$TypeVar.accept(Type.java:1700)
at java.compiler/javax.lang.model.util.AbstractTypeVisitor6.visit(AbstractTypeVisitor6.java:104)
at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.Utils.asTypeElement(Utils.java:1219)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.LinkInfoImpl.setContext(LinkInfoImpl.java:424)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.LinkInfoImpl.<init>(LinkInfoImpl.java:290)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.LinkFactoryImpl.getTypeParameterLink(LinkFactoryImpl.java:135)
at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.links.LinkFactory.getTypeParameterLinks(LinkFactory.java:277)
at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.util.links.LinkFactory.getLink(LinkFactory.java:195)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.HtmlDocletWriter.getLink(HtmlDocletWriter.java:1184)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.HtmlDocletWriter.addPreQualifiedClassLink(HtmlDocletWriter.java:1320)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.HtmlDocletWriter.addPreQualifiedStrongClassLink(HtmlDocletWriter.java:1333)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.AbstractTreeWriter.addPartialInfo(AbstractTreeWriter.java:185)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.AbstractTreeWriter.addLevelInfo(AbstractTreeWriter.java:92)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.AbstractTreeWriter.addLevelInfo(AbstractTreeWriter.java:94)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.AbstractTreeWriter.addTree(AbstractTreeWriter.java:129)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.AbstractTreeWriter.addTree(AbstractTreeWriter.java:112)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.PackageTreeWriter.generatePackageTreeFile(PackageTreeWriter.java:130)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.PackageTreeWriter.generate(PackageTreeWriter.java:107)
at jdk.javadoc/jdk.javadoc.internal.doclets.formats.html.HtmlDoclet.generatePackageFiles(HtmlDoclet.java:340)
at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.AbstractDoclet.startGeneration(AbstractDoclet.java:210)
at jdk.javadoc/jdk.javadoc.internal.doclets.toolkit.AbstractDoclet.run(AbstractDoclet.java:117)
at jdk.javadoc/jdk.javadoc.doclets.StandardDoclet.run(StandardDoclet.java:71)
at jdk.javadoc/jdk.javadoc.internal.tool.Start.parseAndExecute(Start.java:604)
at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:424)
at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:341)
at jdk.javadoc/jdk.javadoc.internal.tool.Main.execute(Main.java:63)
at jdk.javadoc/jdk.javadoc.internal.tool.Main.main(Main.java:52)