[no-this-alias] does not detect all cases of aliasing this to local variable

[fushi]

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have read the FAQ and my problem is not listed.

Repro

{
  "rules": {
   "@typescript-eslint/no-this-alias": "error",
  }
}

let that;

that = this;

Expected Result

3:5 error Unexpected aliasing of 'this' to local variable @typescript-eslint/no-this-alias

Actual Result

No error or warning.

Additional Info

https://github.com/sindresorhus/eslint-plugin-unicorn catches this with unicorn/no-this-assignment

Versions

package	version
@typescript-eslint/eslint-plugin	15.5.0
@typescript-eslint/parser	15.5.0
TypeScript	4.6.2
ESLint	8.9.0
node	16.13.2

[juank1809]

This is because the rule only handles the case when 'this' is being assigned in a variable declaration and ignoring when you are assigning it to a variable already declared

I'd be glad to submit a PR to this issue.

[JoshuaKGoldberg]

Hmm, I'm a little worried that this would cause new rule reports in cases that are actually fine in the spirit of the rule. Vaguely:

class SomeClass {
  doThing(target, useSelf) {
    target.work(useSelf ? target : this);
  }
}

I wonder if there should be an option around this behavior?

I'll defer to @bradzacher - it seems fine to me, but just wanted to raise.

[bradzacher]

Based on the current PR #4718 - this wouldn't match on that junk.
Specifically it'll just be handling thing = this - the AssignmentExpression case.