When compiling overlay sources such as for jsr166 the compiler crashes with this stack trace, instead javac should display a suitable error and fail gracefully. A reproducer attached.

An exception has occurred in the compiler (9-internal). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program and the following diagnostic in your report. Thank you.
java.lang.AssertionError
at com.sun.tools.javac.util.Assert.error(jdk.compiler@9-internal/Assert.java:155)
at com.sun.tools.javac.util.Assert.checkNull(jdk.compiler@9-internal/Assert.java:54)
at com.sun.tools.javac.code.Symtab.enterModule(jdk.compiler@9-internal/Symtab.java:753)
at com.sun.tools.javac.comp.Modules.enterModule(jdk.compiler@9-internal/Modules.java:257)
at com.sun.tools.javac.comp.Modules.enterModules(jdk.compiler@9-internal/Modules.java:235)
at com.sun.tools.javac.comp.Modules.enter(jdk.compiler@9-internal/Modules.java:203)
at com.sun.tools.javac.main.JavaCompiler.readSourceFile(jdk.compiler@9-internal/JavaCompiler.java:816)
at com.sun.tools.javac.main.JavaCompiler.readSourceFile(jdk.compiler@9-internal/JavaCompiler.java:778)
at com.sun.tools.javac.main.JavaCompiler.access$100(jdk.compiler@9-internal/JavaCompiler.java:97)
at com.sun.tools.javac.main.JavaCompiler$1.complete(jdk.compiler@9-internal/JavaCompiler.java:339)
at com.sun.tools.javac.code.ClassFinder.fillIn(jdk.compiler@9-internal/ClassFinder.java:362)
at com.sun.tools.javac.code.ModuleFinder.lambda$findSingleModule$0(jdk.compiler@9-internal/ModuleFinder.java:206)
at com.sun.tools.javac.code.Symbol.complete(jdk.compiler@9-internal/Symbol.java:601)
at com.sun.tools.javac.comp.Modules.setCompilationUnitModules(jdk.compiler@9-internal/Modules.java:361)
at com.sun.tools.javac.comp.Modules.enter(jdk.compiler@9-internal/Modules.java:205)
at com.sun.tools.javac.main.JavaCompiler.initModules(jdk.compiler@9-internal/JavaCompiler.java:1020)
at com.sun.tools.javac.main.JavaCompiler.initModules(jdk.compiler@9-internal/JavaCompiler.java:1012)
at com.sun.tools.javac.main.JavaCompiler.compile(jdk.compiler@9-internal/JavaCompiler.java:906)
at com.sun.tools.javac.main.Main.compile(jdk.compiler@9-internal/Main.java:261)
at com.sun.tools.javac.main.Main.compile(jdk.compiler@9-internal/Main.java:143)
at com.sun.tools.javac.Main.compile(jdk.compiler@9-internal/Main.java:55)
at com.sun.tools.javac.Main.main(jdk.compiler@9-internal/Main.java:41)