Upon building langtools and running javac with no arguments like this:

/opt/JDK/8/re/b20/bin/java -cp build/classes com.sun.tools.javac.Main

I always get this:

Usage: javac <options> <source files>
where possible options include:
  -g Generate all debugging info
  -g:none Generate no debugging info
  -g:{lines,vars,source} Generate only some debugging info
  -nowarn Generate no warnings
  -verbose Output messages about what the compiler is doing
  -deprecation Output source locations where deprecated APIs are used
  -classpath <path> Specify where to find user class files and annotation processors
  -cp <path> Specify where to find user class files and annotation processors
  -sourcepath <path> Specify where to find input source files
  -bootclasspath <path> Override location of bootstrap class files
  -extdirs <dirs> Override location of installed extensions
  -endorseddirs <dirs> Override location of endorsed standards path
  -proc:{none,only} Control whether annotation processing and/or compilation is done.
  -processor <class1>[,<class2>,<class3>...] Names of the annotation processors to run; bypasses default discovery process
  -processorpath <path> Specify where to find annotation processors
  -parameters Generate metadata for reflection on method parameters
  -d <directory> Specify where to place generated class files
  -s <directory> Specify where to place generated source files
  -h <directory> Specify where to place generated native header files
  -implicit:{none,class} Specify whether or not to generate class files for implicitly referenced files
  -encoding <encoding> Specify character encoding used by source files
  -source <release> Provide source compatibility with specified release
  -target <release> Generate class files for specific VM version
  -profile <profile> Check that API used is available in the specified profile
  -version Version information
  -help Print a synopsis of standard options
  -Akey[=value] Options to pass to annotation processors
  -X Print a synopsis of nonstandard options
  -J<flag> Pass <flag> directly to the runtime system
  -Werror Terminate compilation if warnings occur
  @<filename> Read options and filenames from file

Exception in thread "main" java.lang.IllegalArgumentException
at com.sun.tools.javac.main.OptionHelper$GrumpyHelper.put(OptionHelper.java:96)
at com.sun.tools.javac.main.Option.process(Option.java:670)
at com.sun.tools.javac.main.Option.process(Option.java:680)
at com.sun.tools.javac.main.Option$18.process(Option.java:308)
at com.sun.tools.javac.main.Main.compile(Main.java:160)
at com.sun.tools.javac.main.Main.compile(Main.java:140)
at com.sun.tools.javac.Main.compile(Main.java:56)
at com.sun.tools.javac.Main.main(Main.java:42)

This started with the option processing refactoring.

Also reproducible with the official JDK's 9 javac launcher.