Fix: no-extra-parens false positive with let identifier in for-loop

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[x] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Tell us about your environment

• ESLint Version: v7.17.0
• Node Version: v12.18.4
• npm Version: v6.14.6

What parser (default, @babel/eslint-parser, @typescript-eslint/parser, etc.) are you using?

default

Please show your full configuration:

Configuration

module.exports = {
  parserOptions: {
    ecmaVersion: 2015
  }
};

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

Online Demo

/* eslint no-extra-parens: error */

for ((let)[a].b = 1;;);

What did you expect to happen?

No errors. An expression as ForStatement#init cannot start with let[

IterationStatement in the spec:

for ( [lookahead ≠ let [] Expression[~In, ?Yield, ?Await]opt ; Expression[+In, ?Yield, ?Await]opt ; Expression[+In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return]

If for-loop initializer starts with let[, it can be either a let declaration with an array pattern, or a syntax error.

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

  3:6  error  Unnecessary parentheses around expression  no-extra-parens

autofix:

/* eslint no-extra-parens: error */

for (let[a].b = 1;;);

  3:12  error  Parsing error: Complex binding patterns require an initialization value

What changes did you make? (Give an overview)

Fixed the no-extra-parens rule to not report parens inside a for loop initializer expression if removing those parens would make the expression to start with let[.

Is there anything you'd like reviewers to focus on?

The similar logic for for-in and for-of loops is slightly wrong, I'll fix that in another PR.

[btmills]

Tests are very thorough, LGTM