NPE in AbstractReturnValueIgnored

[Stephan202]

Consider this class:

final class Test {
    private final Runnable r = () -> String.valueOf("");
}

Running Error Prone with default settings yields:

Test.java:2: error: An unhandled exception was thrown by the Error Prone static analysis plugin.
    private final Runnable r = () -> String.valueOf("");
                                                   ^
     Please report this at https://github.com/google/error-prone/issues/new and include the following:
  
     error-prone version: 2.0.21
     Stack Trace:
     java.lang.NullPointerException
  	at com.google.errorprone.matchers.IsLastStatementInBlock.matches(IsLastStatementInBlock.java:31)
  	at com.google.errorprone.matchers.IsLastStatementInBlock.matches(IsLastStatementInBlock.java:25)
  	at com.google.errorprone.matchers.Matchers$4.matches(Matchers.java:127)
  	at com.google.errorprone.matchers.Matchers$5.matches(Matchers.java:146)
  	at com.google.errorprone.bugpatterns.AbstractReturnValueIgnored.expectedExceptionTest(AbstractReturnValueIgnored.java:163)
  	at com.google.errorprone.matchers.Matchers$3.matches(Matchers.java:112)
  	at com.google.errorprone.matchers.Matchers$4.matches(Matchers.java:127)
  	at com.google.errorprone.bugpatterns.AbstractReturnValueIgnored.matchMethodInvocation(AbstractReturnValueIgnored.java:80)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitMethodInvocation(ErrorProneScanner.java:907)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitMethodInvocation(ErrorProneScanner.java:146)
  	at com.sun.tools.javac.tree.JCTree$JCMethodInvocation.accept(JCTree.java:1644)
  	at com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:82)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:42)
  	at com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:90)
  	at com.sun.source.util.TreeScanner.visitLambdaExpression(TreeScanner.java:559)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitLambdaExpression(ErrorProneScanner.java:833)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitLambdaExpression(ErrorProneScanner.java:146)
  	at com.sun.tools.javac.tree.JCTree$JCLambda.accept(JCTree.java:1805)
  	at com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:82)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:42)
  	at com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:90)
  	at com.sun.source.util.TreeScanner.visitVariable(TreeScanner.java:223)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitVariable(ErrorProneScanner.java:1156)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitVariable(ErrorProneScanner.java:146)
  	at com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:968)
  	at com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:82)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:42)
  	at com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:90)
  	at com.sun.source.util.TreeScanner.scan(TreeScanner.java:105)
  	at com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:113)
  	at com.sun.source.util.TreeScanner.visitClass(TreeScanner.java:187)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitClass(ErrorProneScanner.java:590)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitClass(ErrorProneScanner.java:146)
  	at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:808)
  	at com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:82)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:42)
  	at com.sun.source.util.TreeScanner.scan(TreeScanner.java:105)
  	at com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:113)
  	at com.sun.source.util.TreeScanner.visitCompilationUnit(TreeScanner.java:144)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:605)
  	at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:146)
  	at com.sun.tools.javac.tree.JCTree$JCCompilationUnit.accept(JCTree.java:591)
  	at com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:56)
  	at com.google.errorprone.scanner.Scanner.scan(Scanner.java:64)
  	at com.google.errorprone.scanner.ErrorProneScannerTransformer.apply(ErrorProneScannerTransformer.java:41)
  	at com.google.errorprone.ErrorProneAnalyzer.finished(ErrorProneAnalyzer.java:145)
  	at com.sun.tools.javac.api.MultiTaskListener.finished(MultiTaskListener.java:120)
  	at com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1425)
  	at com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1374)
  	at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:973)
  	at com.sun.tools.javac.api.JavacTaskImpl.lambda$doCall$0(JavacTaskImpl.java:100)
  	at com.sun.tools.javac.api.JavacTaskImpl.handleExceptions(JavacTaskImpl.java:142)
  	at com.sun.tools.javac.api.JavacTaskImpl.doCall(JavacTaskImpl.java:96)
  	at com.sun.tools.javac.api.JavacTaskImpl.call(JavacTaskImpl.java:90)
  	at com.google.errorprone.BaseErrorProneCompiler.run(BaseErrorProneCompiler.java:137)
  	at com.google.errorprone.BaseErrorProneCompiler.run(BaseErrorProneCompiler.java:108)
  	at com.google.errorprone.ErrorProneCompiler.run(ErrorProneCompiler.java:118)
  	at com.google.errorprone.ErrorProneCompiler.compile(ErrorProneCompiler.java:65)
  	at com.google.errorprone.ErrorProneCompiler.main(ErrorProneCompiler.java:44)
1 error

The issue seems to be triggered because Runnable#run is void, while the lambda expression isn't.

Workarounds:

1. Modify the lambda expression by enclosing the expression in a statement block (i.e., add curly braces).
2. Disable all checks which are a subclass of AbstractReturnValueIgnored (all are enabled by default): -XepCheckReturnValue:OFF -Xep:FutureReturnValueIgnored:OFF -Xep:ReturnValueIgnored:OFF -Xep:RectIntersectReturnValueIgnored:OFF.

[eaftan]

Thanks for the bug report, we'll take a look. Was probably caused by fee27a7.