fix(eslint-plugin): [switch-exhaustiveness-check] better support for intersections, infinite types, non-union values

[auvred]

PR Checklist

• Addresses an existing open issue: fixes Bug: [switch-exhaustiveness-check] requireDefaultForNonUnion false positive on type with one value #8238, fixes Bug: [switch-exhaustiveness-check] invalid fix for union with infinite types #6682
• That issue was marked as accepting prs
• Steps in Contributing were taken

Overview

I decided to fix both issues in one pr, because it was really hard to fix the first one without fixing the second.

This PR introduces better support for all combinations of unions, intersections, infinite types. It also fix suggestions more accurate.

In addition, this PR also includes several fixes for bugs that were present on main but wasn't mentioned in issues:

1. Provide correct fixes for missing unique symbols

Before (playground):

const a = Symbol('a')
const b = Symbol('b')
declare const value: typeof a | typeof b  

switch (value) {
  case a: break;
+ case unique symbol: { throw new Error('Not implemented yet: unique symbol case') }
}

After:

const a = Symbol('a')
const b = Symbol('b')
declare const value: typeof a | typeof b  

switch (value) {
  case a: break;
+ case b: { throw new Error('Not implemented yet: b case') }
}

2. Support intersections that includes literals

Before (playground):

declare const value: ('aa' & {a:1}) | 'bb'
switch (value) {
//      ^ Switch is not exhaustive. Cases not matched: "aa" & { a: 1; }
  case 'aa': break;
  case 'bb': break;
}

After (no false positive):

declare const value: ('aa' & {a:1}) | 'bb'
switch (value) {
  case 'aa': break;
  case 'bb': break;
}

3. Consistently add a default branch both for bare infinite types and unions of infinite types

Before (playground):

declare const value: number
switch (value) {
  case 1: break;
+ default: { throw new Error('default case') }
}


declare const value: number | string
switch (value) {
  case 1: break;
+ case string: { throw new Error('Not implemented yet: string case') }
+ case number: { throw new Error('Not implemented yet: number case') }
}

After:

declare const value: number
switch (value) {
  case 1: break;
+ default: { throw new Error('default case') }
}


declare const value: number | string
switch (value) {
  case 1: break;
+ default: { throw new Error('default case') }
}

---

I've removed isUnion from SwitchMetadata to avoid confusion: we shouldn't treat unions and non-unions differently. We should only care whether a type contains non-literal types or not

[typescript-eslint]

Thanks for the PR, @auvred!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

---

🙏 Please, if you or your company is finding typescript-eslint valuable, help us sustain the project by sponsoring it transparently on https://opencollective.com/typescript-eslint.

[netlify]

✅ Deploy Preview for typescript-eslint ready!

Name	Link
🔨 Latest commit	e343dce
🔍 Latest deploy log	https://app.netlify.com/sites/typescript-eslint/deploys/65a3d4bd5fd1b300085b2ae3
😎 Deploy Preview	https://deploy-preview-8250--typescript-eslint.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 98 (🟢 up 2 from production) Accessibility: 100 (no change from production) Best Practices: 92 (no change from production) SEO: 98 (no change from production) PWA: 80 (no change from production) View the detailed breakdown and full score reports

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[JoshuaKGoldberg]

What a lovely, thoroughly tested PR! Thanks for sending this in! 👏

[JoshuaKGoldberg]

[Non-Actionable] This feels like something we'd want a shared utility for... and, ha, I filed an issue for this back in August! #7466 (comment) -> JoshuaKGoldberg/ts-api-utils#258

Not requesting changes here. I'll go ahead and send that PR to ts-api-utils and then follow up in this repo.

[JoshuaKGoldberg]

JoshuaKGoldberg/ts-api-utils#374