Add no-duplicate-case documentation note about ignoring side effects

Environment

Node version: v16.13.1 npm version: 7.13.0 Local ESLint version: v8.7.0 Global ESLint version: N/A Operating System: Windows 10 Home

What parser are you using?

@typescript-eslint/parser

What did you do?

Configuration

{
	"parser": "@typescript-eslint/parser",
	"parserOptions": {
		"jsx": true,
		"useJSXTextNode": true,
		"ecmaVersion": 2018,
		"sourceType": "module",
		"project": "./tsconfig.json"
	},
	"plugins": ["@typescript-eslint", "prettier"],
	"extends": ["eslint:recommended"],
	"rules": {}
}

let x = 0;
switch (1) {
	case x++: {
		break;
	}
	case x++: {
		break;
	}
}

What did you expect to happen?

Since the code has side effects, the cases being checked are not identical/duplicate, so no error should be generated.

What actually happened?

The code generated a false-positive error on cases that are not duplicate.

Participation

• I am willing to submit a pull request for this issue.

Additional comments

I realise this is a very niche case, I found this while enabling eslint on unit tests for a compiler, hence the code testing special edge case behaviours.

[mdjermanovic]

Hi @Dionysusnu, thanks for the issue!

I can reproduce this with the default parser as well: online demo.

This is indeed not a duplicate, but I think it’s a good case for eslint-disable comments. We could add “When Not To Use It” section in the docs for this rule, mentioning side effects like in no-dupe-else-if rule docs. @eslint/eslint-tsc thoughts?

[mdjermanovic]

I marked this as an accepted issue to update the docs, and prepared PR https://github.com/eslint/eslint/pull/15563