Fix: no-extra-parens false positives for variables called "let"

[not-an-aardvark]

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

[x] Bug fix

Tell us about your environment

• ESLint Version: master
• Node Version: 8.1.2
• npm Version: 5.0.3

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

default

Please show your full configuration:

rules:
  no-extra-parens: error

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

for ((let) in foo); // iterating a variable called "let" over "foo"
for ((let)[foo] in bar); // assigns to the foo property of the "let" variable in a loop
for ((let[foo]) in bar); // assigns to the foo property of the "let" variable in a loop

What did you expect to happen?

I expected eslint --fix to not change the behavior of the code.

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

The code was autofixed to:

for (let in foo); // invalid syntax
for (let[foo] in bar); // destructuring variable assignment (behavior change)
for (let[foo] in bar); // destructuring variable assignment (behavior change)

What changes did you make? (Give an overview)

"let" is unusual because it's sometimes parsed as a variable declaration keyword, and sometimes as an identifier for a variable. This commit fixes some bugs in the no-extra-parens rule where parentheses are unnecessary for most variable names, but are necessary when the variable is called "let".

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

Nothing in particular

[eslintbot]

LGTM

[platinumazure]

LGTM, just one question: Should this also apply to const as well?

[not-an-aardvark]

No, because unlike let, const is not a valid variable name.