False UnusedVariable warning for method parameters only used in overridden implementations

[mlopatkin]

Consider the following example:

public class App {
    private static class Base {
        protected void doStuff(String usedInDescendants) {}
    }

    private static class Descendant extends Base {
        @Override
        protected void doStuff(String actuallyUsed) {
            System.out.println(actuallyUsed);
        }
    }

    public static void main(String[] args) {
        Base b = new Descendant();

        b.doStuff("some string");
    }
}

Expected result: no warnings

Actual result: ErrorProne complains about usedInDescendants parameter of App.Base.doStuff being unused. However, this parameter is used by the overridden method in Descendant class and cannot be easily removed.

There are several workarounds, but they aren't universally applicable:

• Increase the visibility of the base class - gives the type unnecessary exposure
• Make the method abstract - then the base class cannot provide a default implementation.

Even converting the Base to an interface with default method still triggers the warning.

A full reproducer can be found at
https://github.com/mlopatkin/errorprone-issue-reproducers/tree/trunk/unused-variable-overloaded-method

Tested with ErrorProne 2.27.1, 2.24.1 on Java 11 and Java 21.

[cpovirk]

(Is this fixed by #4433, which started off being about a regression test but seems to have developed into an actual fix?)

[mlopatkin]

Can confirm that the issue is fixed in 2.29.0. Thank you for solving it so quickly!

[mlopatkin]

As of 2.29.1, the warning still happens when the descendant is an anonymous class:
https://github.com/mlopatkin/errorprone-issue-reproducers/blob/trunk/unused-variable-overloaded-method/src/main/java/com/example/AnonymousDescendant.java