fix(type-utils): union types always being marked as readonly

[RebeccaStevens]

PR Checklist

• Addresses an existing issue: fixes [isTypeReadonly] Unions are always marked as readonly #4418
• That issue was marked as accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

isTypeReadonly now checks unions properly.

Note: This PR builds off #4416

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[typescript-eslint]

Thanks for the PR, @RebeccaStevens!

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!

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[codecov]

Codecov Report

Merging #4419 (727d999) into main (ef3147c) will decrease coverage by 1.52%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4419      +/-   ##
==========================================
- Coverage   94.23%   92.70%   -1.53%     
==========================================
  Files         169      161       -8     
  Lines        9448     8144    -1304     
  Branches     2948     2612     -336     
==========================================
- Hits         8903     7550    -1353     
- Misses        321      395      +74     
+ Partials      224      199      -25     

Flag	Coverage Δ
unittest	92.70% <ø> (-1.53%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/type-utils/src/isTypeReadonly.ts	78.57% <ø> (ø)
...ript-estree/src/create-program/createSourceFile.ts
...ckages/typescript-estree/src/jsx/xhtml-entities.ts
...lugin-internal/src/rules/plugin-test-formatting.ts
...-internal/src/rules/no-typescript-estree-import.ts
packages/typescript-estree/src/convert.ts
...ges/typescript-estree/src/create-program/shared.ts
packages/typescript-estree/src/node-utils.ts
...estree/src/create-program/createIsolatedProgram.ts
...pt-estree/src/create-program/createWatchProgram.ts
... and 27 more

[JoshuaKGoldberg]

LGTM! I'm up for merging this in as-is, but feel mildly positive about suggested changes. What about you?

If nobody is opinionated after a few days + the next time I take a look, I can just merge :)

[JoshuaKGoldberg]

Hmm, this is essentially the same as the code at the beginning of isUnsafeAssignment.

typescript-eslint/packages/type-utils/tests/isUnsafeAssignment.test.ts

Line 16 in a9eb0b9

const { ast, services } = parseForESLint(code, {

Since we're starting to establish a precedent of unit testing these type utilities directly, I think we'll want to make a standard way to generate the AST & node maps for them. What do you think about making a standard util like this?:

const { body, checker, esTreeNodeToTSNodeMap } = parseForUtilTest(code);

[bradzacher]

I merged your test PR first - just needs the conflict resolved and we can merge this!

[bradzacher]

thanks for fixing this!