4.18.2 - "Cannot read property 'type' of undefined" in no-unused-vars when checking flow type definitions

Tell us about your environment

• ESLint Version: 4.18.2 (it works in 4.18.1)!
• Node Version: 9.8.0
• yarn Version: 1.5.1

What parser (default, Babel-ESLint, etc.) are you using? babel-eslint@8.2.2

Please show your full configuration:

Configuration

module.exports = {
  parser: 'babel-eslint',
  rules: {
    'no-unused-vars': 'error'
  }
};

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

The issue only occures for files where a result type is defined for an arrow function:

// @flow

type Type = null;
// Defining result type causes the crash
export const f = (): Type => null;
// This would work
export const f2 = () => null;

eslint source.js

What actually happened? Please include the actual, raw output from ESLint.

Cannot read property 'type' of undefined
TypeError: Cannot read property 'type' of undefined
    at isForInRef (/Users/tgs/projects/brainly/brainly-frontend/node_modules/eslint/lib/rules/no-unused-vars.js:406:24)
    at variable.references.some.ref (/Users/tgs/projects/brainly/brainly-frontend/node_modules/eslint/lib/rules/no-unused-vars.js:443:21)
    at Array.some (<anonymous>)
    at isUsedVariable (/Users/tgs/projects/brainly/brainly-frontend/node_modules/eslint/lib/rules/no-unused-vars.js:442:40)
    at collectUnusedVariables (/Users/tgs/projects/brainly/brainly-frontend/node_modules/eslint/lib/rules/no-unused-vars.js:565:26)
    at collectUnusedVariables (/Users/tgs/projects/brainly/brainly-frontend/node_modules/eslint/lib/rules/no-unused-vars.js:572:17)
    at Program:exit (/Users/tgs/projects/brainly/brainly-frontend/node_modules/eslint/lib/rules/no-unused-vars.js:617:36)
    at listeners.(anonymous function).forEach.listener (/Users/tgs/projects/brainly/brainly-frontend/node_modules/eslint/lib/util/safe-emitter.js:47:58)
    at Array.forEach (<anonymous>)
    at Object.emit (/Users/tgs/projects/brainly/brainly-frontend/node_modules/eslint/lib/util/safe-emitter.js:47:38)

It works correctly in 4.18.1!

[tomasz-sodzawiczny]

@platinumazure @not-an-aardvark Ugh, OK, I tried to reproduce that in an isolated setup and I found the issue - it’s not with eslint, it’s problem with yarn workspaces, false alarm.

In case some has the same thing: it seems when using yarn workspaces eslint binary might run against a different babel-eslint than the one defined in your package. Yuck.

Sorry I wasted your time. 😦

[not-an-aardvark]

Closing this issue since it turned out to not be a bug. Thanks for the report regardless – I think we have room for improvement in helping the user debug this type of issue.