no-unused-vars argsIgnorePattern affecting "args": "after-used" since 3.6.0

[danpoltawski]

Tell us about your environment

• ESLint Version: v3.6.0
• Node Version: v6.6.0
• npm Version: 3.10.7

What parser (default, Babel-ESLint, etc.) are you using? deafult

Please show your full configuration: https://github.com/moodle/moodle/blob/master/.eslintrc

What did you do? Please include the actual source code causing the issue.

When attempting to upgrade to eslint 3.6.0, we got some new errors reported by no-unused-vars which should have been ignored by args: 'after-used'. It turned out that argsIgnorePattern is now affecting the result of things which should be ignored by "args": "after-used"

I attempted to isolate the code down to the smallest test case:

Problem file:

/* eslint 'no-unused-vars': ['error', {"args": "after-used", "argsIgnorePattern": "^baz$"}] */

(function(foo, bar, baz) {
        return baz;
})();

Output:

$ eslint problem.js 

/Users/danp/www/eslintest/problem.js
  3:16  error  'bar' is defined but never used  no-unused-vars

✖ 1 problem (1 error, 0 warnings)

Ok file:

/* eslint 'no-unused-vars': ['error', {"args": "after-used"}] */

(function(foo, bar, baz) {
        return baz;
})();

[danpoltawski]

(Note: this was detected because we have our argsIgnorePattern too liberally matching by mistake)

[not-an-aardvark]

This was probably caused by #7143.

I think the issue is that we now check for the last non-ignored parameter (in your case, bar) rather than the last parameter itself (baz). However, we don't account for cases where an ignored parameter is used.

[platinumazure]

Confirmed in the online demo. I think this is something we should consider for a patch release, if possible.

[mysticatea]

Thank you for this issue.

Though I'd like to say this is intentional since baz is ignored, we should fix this issue to prevent a breaking change without major version up.
I will work on this within today (JST).

By the way, I'm interested in the option which warns such access to ignored variables. Maybe disallowUseIgnored. Or a separated rule such as no-use-ignored-vars.

[danpoltawski]

Thanks! As mentioned above - in our case it was caused by a misconfiguration of argsIgnorePattern which matched a lot of args, so I am not sure how much you should be concerned about this being a problem in practice.