Rule "curly" with "multi" has false positive on nested if-for-if, fixer changes code behavior

[drakedevel]

Tell us about your environment

• ESLint Version: 6.8.0
• Node Version: 12.16.0
• npm Version: 6.13.4

This bug also reproduces on ESLint 7.0.0-alpha.1.

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

Please show your full configuration:

Configuration

{
  "rules": {
    "curly": ["warn", "multi"]
  }
}

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

var x = [''];
if (x)
  for (var i in x) {
    if (i > 0)
      console.log(i);
  }
else
  console.log('whoops');

./node_modules/.bin/eslint --fix index.js

What did you expect to happen?
ESLint should make no changes to the code, as the only curly braces present are required.

What actually happened? Please include the actual, raw output from ESLint.
ESLint changes the code to the following:

var x = [''];
if (x)
  for (var i in x) 
    if (i > 0)
      console.log(i);
  
else
  console.log('whoops');

This is incorrect -- the else is now associated with the inner if instead of the outer one. The execution behavior changes accordingly:

$ node index.js 
$ ./node_modules/.bin/eslint index.js

/tmp/eslint-repro/index.js
  3:3  warning  Unnecessary { after 'for-in'  curly

✖ 1 problem (0 errors, 1 warning)
  0 errors and 1 warning potentially fixable with the `--fix` option.

$ ./node_modules/.bin/eslint --fix index.js 
$ node index.js 
whoops

I did a little bit more experimentation: when I replace the for block with a while block, the same issue occurs. However, when I replace the for block with another if, ESLint correctly detects that the curly braces are mandatory.

Are you willing to submit a pull request to fix this bug?
I can if necessary, although the fix is probably much faster for somebody who's already familiar with the relevant code.

[mdjermanovic]

Hi @drakedevel, thanks for the bug report!

Confirmed:

• Eslint Demo
• ASTExplorer with the original code
• ASTExplorer with the fixed code

The outer if has alternate: null in AST for the fixed code.

[mdjermanovic]

I'm working on this.

[drakedevel]

Confirmed fixed for me in 7.0.0-alpha.2, thanks!