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

[tomasz-sodzawiczny]

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!

[ilyavolodin]

When you tested it with 4.18.1 did you use the same version of babel-eslint? Because the last change to no-unused-vars rule (the one that's failing), was done in 4.18.0, so if that change is the cause of the issue, 4.18.1 should also be failing as well.

[not-an-aardvark]

@ilyavolodin If I had to guess, this was caused by the refactoring in core that went into 4.18.2. It might have changed something about how scope analysis is resolved.

[platinumazure]

Incidentally, this seems to be the same error that redux is showing in eslint-canary.

I'm going to do a quick bisect to try to learn more.

[platinumazure]

@tomasz-sodzawiczny I can't reproduce on my machine. Am I doing something wrong?

I created a subdirectory in the eslint repo and created these files:

• .eslintrc.json (N.B. root: true is to avoid pulling in ancestor eslint config files and should not affect test):

  {
      "parser": "babel-eslint",
      "rules": {
          "no-unused-vars": "error"
      },
      "root": true
  }

• file.js:

  // @flow
  
  type Type = null;
  
  export const f = (): Type => null;

• test.sh:

  #!/bin/bash
  
  echo "Running npm install..."
  npm install --quiet
  
  echo "Installing babel-eslint..."
  npm install babel-eslint@8.2.2 --quiet
  
  echo "Linting the test file..."
  
  if node ../bin/eslint.js ./file.js; then
      echo "Linting successful!"
  else
      echo "Linting failed!"
  fi

The last is intended to be a script for bisecting, run after checking out a commit. I've run it on both latest master and on the v4.18.2 tag, with the expectation that linting should fail/crash. Just in case my bash skills are rusty, I've also run ESLint directly, outside the script (using the same node ../bin/eslint.js ./file.js input) and also saw no linting errors in both latest master and v4.18.2 tag.

Have I missed something?

[not-an-aardvark]

Incidentally, this seems to be the same error that redux is showing in eslint-canary.

This is also the error that was occurring in #9762 (where scope analysis with babel-eslint was incorrect when we added the ability to use custom scope analysis). However, aside from this issue I'm not aware of a case where this occurs when using the latest version of babel-eslint.

[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. :(

[platinumazure]

I've created #10100 to add some extra debugging output that might come in handy in similar situations in the future.

[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.