Incorrect dead code elimination with class instanceof

[AshleyScirra]

Using Closure Compiler v20200517

in.js:

window.Class = class MyClass {
	constructor()
	{
		if (this instanceof MyClass)
			console.log("test");
	}
}

new window.Class();

Note if you run this in a browser console it logs "test".

Compile with the following options:

java -jar ./closure-compiler.jar --js in.js --js_output_file out.js --compilation_level ADVANCED --language_in ECMASCRIPT_2015 --language_out ECMASCRIPT_2015

out.js looks like this:

window.a=class{
	constructor(){}
};
new window.a;

Note it has incorrectly removed the console log. Note the class name has been removed, which may have then caused this instanceof MyClass to be identified as false and therefore dead code.

[concavelenz]

Thank you for the report and minimal repro case. This look similiar to another issue @brad4d was looking and maybe has a fix pending for?

[brad4d]

This is similar to another issue that I fixed, but not quite the same.

I've reproduced the problem with the latest version of the compiler code by following the example here.

1. RemoveUnusedCode sees only one reference to MyClass, and it's an instanceof check, so it believes no objects of MyClass get created and replaces the instanceof check with false.
2. Then RemoveUnusedCode removes the MyClass name from the class definition.