FULL PRODUCT VERSION :
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Darwin $HOSTNAME 15.6.0 Darwin Kernel Version 15.6.0: Tue Jan 9 20:12:05 PST 2018; root:xnu-3248.73.5~1/RELEASE_X86_64 x86_64

A DESCRIPTION OF THE PROBLEM :
I have a jar that includes the following classes and interface:

public class Outer {
    public class Inner implements Generic<Outer> { }
}

interface Generic<T> { }

I wanted to make the inner class standalone so that it could be packaged in a separate jar, and I also wanted to maintain backwards compatibility for anyone using both jars. I attempted to do so by creating the following class and compiling it against the original jar.

public class Outer$Inner implements Generic<Outer> { }

This causes the compiler to throw an exception. If I remove the inner class from the original jar then compilation succeeds, but in my case I am not able to modify the original jar until after I have created and distributed the second jar.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Create a file Outer.java with the following content:

public class Outer {
    public class Inner implements Generic<Outer> { }
}

interface Generic<T> { }

2. Compile the code and add it to a jar:

javac Outer.java && jar cvf bug.jar Outer.class Outer\$Inner.class Generic.class

3. Create a file Outer$Inner.java with the following content:

public class Outer$Inner implements Generic<Outer> { }

4. Attempt to compile the second file against the original jar:

javac -cp bug.jar Outer\$Inner.java

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I expected compilation to succeed. (Or at least to fail with a duplicate class error.)
ACTUAL -
The compiler crashed with an exception.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
An exception has occurred in the compiler (9.0.4). 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.NullPointerException
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.visitIdent(Flow.java:2478)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCIdent.accept(JCTree.java:2237)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:393)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1451)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scanExpr(Flow.java:1707)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.visitApply(Flow.java:2331)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodInvocation.accept(JCTree.java:1628)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:393)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1451)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.visitExec(TreeScanner.java:213)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCExpressionStatement.accept(JCTree.java:1446)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:393)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1451)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:57)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.visitBlock(Flow.java:1956)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:1014)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:393)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1451)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.visitMethodDef(Flow.java:1884)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:866)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:393)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1451)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.visitClassDef(Flow.java:1822)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:774)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:393)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1451)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.analyzeTree(Flow.java:2521)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.analyzeTree(Flow.java:2504)
at jdk.compiler/com.sun.tools.javac.comp.Flow.analyzeTree(Flow.java:212)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1389)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1363)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:959)
at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:302)
at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:162)
at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:57)
at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:43)


REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
If you are able to update the original code at the same time, then removing the inner class from the original jar will allow compilation to succeed.