Using a method reference of a varargs method as a BinaryOperator crashes the GWT compiler

[OneGeek]

GWT version: 2.8.1
Browser (with version): N/A
Operating System: Windows 7

---

Description

Trying to use a method reference of a varargs method in a place that expects a BinaryOperator causes the GWT compiler to crash with the following trace:

Compiling module com.ekotrope.TestMethodReferenceArityBug
[ERROR] Unexpected internal compiler error
java.lang.ArrayIndexOutOfBoundsException: 2
	at com.google.gwt.dev.jjs.impl.GwtAstBuilder$AstVisitor.endVisit(GwtAstBuilder.java:1907)
	at org.eclipse.jdt.internal.compiler.ast.ReferenceExpression.traverse(ReferenceExpression.java:946)
	at org.eclipse.jdt.internal.compiler.ast.LocalDeclaration.traverse(LocalDeclaration.java:309)
	at org.eclipse.jdt.internal.compiler.ast.MethodDeclaration.traverse(MethodDeclaration.java:357)
	at org.eclipse.jdt.internal.compiler.ast.TypeDeclaration.traverse(TypeDeclaration.java:1360)
	at com.google.gwt.dev.jjs.impl.GwtAstBuilder.processImpl(GwtAstBuilder.java:3916)
	at com.google.gwt.dev.jjs.impl.GwtAstBuilder.process(GwtAstBuilder.java:3954)
	at com.google.gwt.dev.javac.CompilationStateBuilder$CompileMoreLater$UnitProcessorImpl.process(CompilationStateBuilder.java:129)
	at com.google.gwt.dev.javac.JdtCompiler$CompilerImpl.process(JdtCompiler.java:384)
	at org.eclipse.jdt.internal.compiler.Compiler.processCompiledUnits(Compiler.java:546)
	at org.eclipse.jdt.internal.compiler.Compiler.compile(Compiler.java:458)
	at com.google.gwt.dev.javac.JdtCompiler.doCompile(JdtCompiler.java:1092)
	at com.google.gwt.dev.javac.CompilationStateBuilder$CompileMoreLater.compile(CompilationStateBuilder.java:325)
	at com.google.gwt.dev.javac.CompilationStateBuilder.doBuildFrom(CompilationStateBuilder.java:548)
	at com.google.gwt.dev.javac.CompilationStateBuilder.buildFrom(CompilationStateBuilder.java:479)
	at com.google.gwt.dev.javac.CompilationStateBuilder.buildFrom(CompilationStateBuilder.java:465)
	at com.google.gwt.dev.cfg.ModuleDef.getCompilationState(ModuleDef.java:423)
	at com.google.gwt.dev.Precompile.precompile(Precompile.java:222)
	at com.google.gwt.dev.Precompile.precompile(Precompile.java:202)
	at com.google.gwt.dev.Precompile.precompile(Precompile.java:143)
	at com.google.gwt.dev.Compiler.compile(Compiler.java:204)
	at com.google.gwt.dev.Compiler.compile(Compiler.java:155)
	at com.google.gwt.dev.Compiler.compile(Compiler.java:144)
	at com.google.gwt.dev.Compiler$1.run(Compiler.java:118)
	at com.google.gwt.dev.CompileTaskRunner.doRun(CompileTaskRunner.java:55)
	at com.google.gwt.dev.CompileTaskRunner.runWithAppropriateLogger(CompileTaskRunner.java:50)
	at com.google.gwt.dev.Compiler.main(Compiler.java:125)

Steps to reproduce

public class TestMethodReferenceArityBug implements EntryPoint
{
    @Override
    public void onModuleLoad()
    {
        BinaryOperator<Object> op = this::foo;
    }
    
    private Object foo(Object... unused)
    {
        return null;
    } 
}

Known workarounds

Wrap the call in a lambda.

BinaryOperator<Object> op = (a, b) -> foo(a,b);

[rluble]

Thanks for the clear repro case.

The construction used for method references is quite fragile and (as you noted) not correct when there are varargs. The logic in GwtAstBuilder.AstVisitor.endVisit(ReferenceExpression x, BlockScope blockScope) needs to be cleaned up.

I have started a patch here. It would be nice if someone adds tests for vararg constructor method references (with inner classes). The varargs can be in the method reference or in the functional method or both.

It would be nice to see how many of these cases fail.