UnusedNestedClass leads to NullPointerException (with repro)

[zimmi]

Description of the problem / feature request:

I'm trying to export a package containing a nested static inner class through module-info.java. This seems to trigger the UnusedNestedClass rule (which it shouldn't, as the inner class is public and therefore exported, but this is probably a separate problem). This leads to the following stacktrace:

module-info.java:[1,1] An unhandled exception was thrown by the Error Prone static analysis plugin.
     Please report this at https://github.com/google/error-prone/issues/new and include the following:
  
     error-prone version: 2.3.3
     BugPattern: UnusedNestedClass
     Stack Trace:
     java.lang.NullPointerException
  	at com.google.errorprone.util.ASTHelpers.enclosingClass(ASTHelpers.java:880)
  	at com.google.errorprone.bugpatterns.UnusedNestedClass$ClassUsageScanner.handle(UnusedNestedClass.java:116)
  	at com.google.errorprone.bugpatterns.UnusedNestedClass$ClassUsageScanner.visitMemberSelect(UnusedNestedClass.java:105)
  	at com.google.errorprone.bugpatterns.UnusedNestedClass$ClassUsageScanner.visitMemberSelect(UnusedNestedClass.java:87)
  	at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCFieldAccess.accept(JCTree.java:2118)
  	at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
  	at jdk.compiler/com.sun.source.util.TreeScanner.visitRequires(TreeScanner.java:880)
  	at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCRequires.accept(JCTree.java:2852)
  	at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
  	at jdk.compiler/com.sun.source.util.TreeScanner.scan(TreeScanner.java:105)
  	at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:113)
  	at jdk.compiler/com.sun.source.util.TreeScanner.visitModule(TreeScanner.java:853)
  	at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCModuleDecl.accept(JCTree.java:2701)
  	at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
  	at jdk.compiler/com.sun.source.util.TreeScanner.scan(TreeScanner.java:105)
  	at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:113)
  	at jdk.compiler/com.sun.source.util.TreeScanner.visitCompilationUnit(TreeScanner.java:144)
  	at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCCompilationUnit.accept(JCTree.java:591)
  	at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:56)
  	at com.google.errorprone.bugpatterns.UnusedNestedClass.matchCompilationUnit(UnusedNestedClass.java:62)
  	at com.google.errorprone.scanner.ErrorProneScanner.processMatchers(ErrorProneScanner.java:433)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:541)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:150)
  	at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCCompilationUnit.accept(JCTree.java:591)
  	at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:56)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:55)
  	at com.google.errorprone.scanner.ErrorProneScannerTransformer.apply(ErrorProneScannerTransformer.java:43)
  	at com.google.errorprone.ErrorProneAnalyzer.finished(ErrorProneAnalyzer.java:147)
  	at jdk.compiler/com.sun.tools.javac.api.MultiTaskListener.finished(MultiTaskListener.java:132)
  	at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1418)
  	at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1365)
  	at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:960)
  	at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.lambda$doCall$0(JavacTaskImpl.java:104)
  	at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.handleExceptions(JavacTaskImpl.java:147)
  	at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.doCall(JavacTaskImpl.java:100)
  	at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.call(JavacTaskImpl.java:94)
  	at org.codehaus.plexus.compiler.javac.JavaxToolsCompiler.compileInProcess(JavaxToolsCompiler.java:126)
  	at org.codehaus.plexus.compiler.javac.JavacCompiler.performCompile(JavacCompiler.java:174)
  	at org.apache.maven.plugin.compiler.AbstractCompilerMojo.execute(AbstractCompilerMojo.java:1129)
  	at org.apache.maven.plugin.compiler.CompilerMojo.execute(CompilerMojo.java:188)
  	at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:134)
  	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:207)
  	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
  	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
  	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:116)
  	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:80)
  	at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51)
  	at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
  	at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:307)
  	at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:193)
  	at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:106)
  	at org.apache.maven.cli.MavenCli.execute(MavenCli.java:863)
  	at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:288)
  	at org.apache.maven.cli.MavenCli.main(MavenCli.java:199)
  	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
  	at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
  	at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
  	at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
  	at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

1. Clone https://github.com/zimmi/unused-nested-repro
2. Run mvn clean package to see the error message
3. Adding exports org.example; to module-info.java does not make a difference to trigger the error.
4. Removing the line requires java.base; from module-info.java makes it work somehow (adding any other requires clause seems to also make it fail).

What version of Error Prone are you using?

2.3.3

java --version:

openjdk 11.0.2 2019-01-15
OpenJDK Runtime Environment (build 11.0.2+9-Debian-3)
OpenJDK 64-Bit Server VM (build 11.0.2+9-Debian-3, mixed mode, sharing)

Have you found anything relevant by searching the web?

No.

[tapacuk93]

Catched same problem.

[cpovirk]

I think I have a fix.