feat(eslint-plugin): [no-redundant-type-constituents] use assignability checking for redundancy checks #10744

mdm317 · 2025-01-29T16:14:20Z

PR Checklist

Addresses an existing open issue: fixes Enhancement: [no-redundant-type-constituents] Use assignability checking for redundancy checks #7742
That issue was marked as accepting prs
Steps in Contributing were taken

Overview

even cases where they're deeply equal, but declared as different object types.

This feature seems to overlap with this rule no-duplicate-type-constituents so I'm not sure whether to include it.
For now, I'll remove it and proceed.

deeply equal

type ShallowEqual = number | number;
type DeepEqual = { a: string } | { a: string };

Union
If x is assignable to y, then in the union x∣y, x is redundant and can be removed.
However, due to TypeScript's Excess Property Checks feature, this logic did not work as expected.

For example, consider the union type { a: 1 } | { a: 1, b: 1 }. Since { a: 1, b: 1 } is assignable to { a: 1 }
So { a: 1, b: 1 } becomes redundant and can be removed.

However, if { a: 1, b: 1 } is removed, the remaining type { a: 1 } exhibits different behavior when declaring values.
For example, { a: 1 } alone can only be assigned to variables of type { a: 1 }, whereas { a: 1 } | { a: 1, b: 1 } allows the declaration of both { a: 1 } and { a: 1, b: 1 }

Example code

type Foo = { a: 1 } | { a: 1, b: 1 }
const foo: Foo = { a: 1 }
const bar: Foo = { a: 1, b: 1 }

type Foo2 = { a: 1 } 
const foo2: Foo2 = { a: 1 }
const bar2: Foo2 = { a: 1, b: 1 } //error

So, before checking whether one type is assignable to another, I first verified that both objects have the same set of keys. Only then did I proceed with the assignability check.
For A | B, i checks if A and B have the same keys. If they do, it then checks assignability.

Union Types with Intersection Types

Union with a Single Intersection:
- If a union type contains an intersection, like A | (B & C & D), the intersection (B & C & D) is treated as a single object type.
Union within an Intersection:
- If an intersection includes a union, like A | (B & (C | D)), TypeScript simplifies the intersection into a union of intersections. B & (C | D) is equivalent to B & C | B & D,
- This means the assignability check becomes: Is A assignable to (B & C) | (B & D)

Intersection
Unlike union types, intersection types did not need excess property checks.
A & B in this case if A is assignable to B B is rebundant.

If there is no union type within the intersection, I proceed the assignability check. A & B
If there is union type, I check if every type in the union is compatible with the intersection and follows the same rules.
A & ( B | C)

A >= B, A >= C  Then  A >= (B | C)
B >= A, C >= A  Then   (B | C) >= A

typescript-eslint · 2025-01-29T16:14:24Z

Thanks for the PR, @mdm317!

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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nx-cloud · 2025-01-29T16:15:19Z

View your CI Pipeline Execution ↗ for commit acb39d8

Command	Status	Duration	Result
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`nx run-many -t lint`	✅ Succeeded	3m 29s	View ↗
`nx run-many -t typecheck`	✅ Succeeded	2m 14s	View ↗
`nx test eslint-plugin-internal --coverage=false`	✅ Succeeded	4s	View ↗
`nx run types:build`	✅ Succeeded	5s	View ↗
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☁️ Nx Cloud last updated this comment at 2025-11-08 04:52:01 UTC

codecov · 2025-01-29T16:31:30Z

Codecov Report

❌ Patch coverage is 92.07746% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.61%. Comparing base (ea2ee6b) to head (acb39d8).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
...plugin/src/rules/no-redundant-type-constituents.ts	92.38%	42 Missing and 1 partial ⚠️
...escript-estree/src/semantic-or-syntactic-errors.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10744      +/-   ##
==========================================
- Coverage   90.66%   90.61%   -0.05%     
==========================================
  Files         518      518              
  Lines       52435    52733     +298     
  Branches     8686     8771      +85     
==========================================
+ Hits        47541    47785     +244     
- Misses       4880     4933      +53     
- Partials       14       15       +1

Flag	Coverage Δ
unittest	`90.61% <92.07%> (-0.05%)`	⬇️

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

Files with missing lines	Coverage Δ
...ages/eslint-plugin/src/rules/no-unsafe-argument.ts	`93.24% <100.00%> (ø)`
...nt-plugin/src/rules/related-getter-setter-pairs.ts	`100.00% <ø> (ø)`
...escript-estree/src/semantic-or-syntactic-errors.ts	`2.32% <0.00%> (ø)`
...plugin/src/rules/no-redundant-type-constituents.ts	`93.41% <92.38%> (-4.23%)`	⬇️

... and 3 files with indirect coverage changes

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
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we can not know typename {a : 2 } using typetostring type B = {a : 2}; type T = { a: 1 } | B

…ame keys.

JoshuaKGoldberg · 2025-03-03T13:43:15Z

👋 Ping @mdm317, are you planning on un-draft ing this soon? We'd prefer not to keep drafts around for too long in case someone else would want to send one too.

mdm317 · 2025-03-03T15:28:34Z

Sorry for the delay. This issue was more challenging than I expected.
I'll try to finish it within two days. If it takes longer, I'll close the PR and reopen it once it's resolved.
Moving forward, if I anticipate a delay, I'll close it first and reopen it when it's ready.

packages/eslint-plugin/src/rules/related-getter-setter-pairs.ts

packages/scope-manager/tests/test-utils/serializers/TSESTreeNode.ts

JoshuaKGoldberg

OK! Sorry for the delay in reviewing. This is a really tricky one 😅.

I think the feature request is a lot trickier than the test cases here currently capture. It'll have to handle a lot of cases, including optional properties, interfaces extending each other, and so on. I suspect the core implementation will have to adjust a bunch - so left a few suggestions on how to reduce the amount of work done. Hope it's helpful, and let me know if you have questions! 🙌

packages/eslint-plugin/src/rules/no-redundant-type-constituents.ts

packages/eslint-plugin/tests/rules/no-redundant-type-constituents.test.ts

packages/eslint-plugin/src/rules/no-redundant-type-constituents.ts

mdm317 · 2025-03-20T15:32:29Z

Thank you for your review!
I'll try implementing it according to your suggestion.

packages/eslint-plugin/src/rules/no-redundant-type-constituents.ts

JoshuaKGoldberg

🥳 Whoohoo! Thanks so much for bearing with me on reviews and making so many refactors. This is a lot of code by necessity, but it's roughly as clean as I think it can get. Nice job!

Just to be safe I'd like to get another review from someone on @typescript-eslint/triage-team if possible (though we are swamped, and I don't want to wait beyond the end of this month).

bradzacher · 2025-11-23T11:13:27Z

packages/eslint-plugin/src/rules/no-redundant-type-constituents.ts

+  checker: ts.TypeChecker,
+  depth = 0,
+): boolean {
+  if (depth > 10) {


another way to do this is to keep track of "seen" types.
If the type is already in the set then it's recursive.

@dbarabashh

| datasource | package | from | to | | ---------- | -------------------------------- | ------ | ------ | | npm | @typescript-eslint/eslint-plugin | 8.47.0 | 8.48.0 | | npm | @typescript-eslint/parser | 8.47.0 | 8.48.0 | ## [v8.48.0](https://github.com/typescript-eslint/typescript-eslint/blob/HEAD/packages/eslint-plugin/CHANGELOG.md#8480-2025-11-24) ##### 🚀 Features - **eslint-plugin:** \[no-redundant-type-constituents] use assignability checking for redundancy checks ([#10744](typescript-eslint/typescript-eslint#10744)) ##### 🩹 Fixes - **typescript-estree:** disallow binding patterns in parameter properties ([#11760](typescript-eslint/typescript-eslint#11760)) - **eslint-plugin:** \[consistent-generic-constructors] ignore when constructor is typed array ([#10477](typescript-eslint/typescript-eslint#10477)) ##### ❤️ Thank You - Dima Barabash [@dbarabashh](https://github.com/dbarabashh) - JamesHenry [@JamesHenry](https://github.com/JamesHenry) - Josh Goldberg - mdm317 [@gen-ip-1](https://github.com/gen-ip-1) You can read about our [versioning strategy](https://typescript-eslint.io/users/versioning) and [releases](https://typescript-eslint.io/users/releases) on our website.

@dbarabashh

| datasource | package | from | to | | ---------- | -------------------------------- | ------ | ------ | | npm | @typescript-eslint/eslint-plugin | 8.47.0 | 8.48.0 | | npm | @typescript-eslint/parser | 8.47.0 | 8.48.0 | ## [v8.48.0](https://github.com/typescript-eslint/typescript-eslint/blob/HEAD/packages/eslint-plugin/CHANGELOG.md#8480-2025-11-24) ##### 🚀 Features - **eslint-plugin:** \[no-redundant-type-constituents] use assignability checking for redundancy checks ([#10744](typescript-eslint/typescript-eslint#10744)) ##### 🩹 Fixes - **typescript-estree:** disallow binding patterns in parameter properties ([#11760](typescript-eslint/typescript-eslint#11760)) - **eslint-plugin:** \[consistent-generic-constructors] ignore when constructor is typed array ([#10477](typescript-eslint/typescript-eslint#10477)) ##### ❤️ Thank You - Dima Barabash [@dbarabashh](https://github.com/dbarabashh) - JamesHenry [@JamesHenry](https://github.com/JamesHenry) - Josh Goldberg - mdm317 [@gen-ip-1](https://github.com/gen-ip-1) You can read about our [versioning strategy](https://typescript-eslint.io/users/versioning) and [releases](https://typescript-eslint.io/users/releases) on our website.

JoshuaKGoldberg · 2025-12-01T14:50:23Z

This change is very exciting, but has a lot more edge cases and implications than we realized 😬. Reverted in #11812 - which has a list of issues. #7742 is now open again with a note.

…; re-enable the bugged lint rule: eslint-plugin: revert: [no-redundant-type-constituents] use assignability checking for redundancy checks (typescript-eslint/typescript-eslint#10744) (typescript-eslint/typescript-eslint#11812)

…; re-enable the bugged lint rule: - eslint-plugin: revert: [no-redundant-type-constituents] use assignability checking for redundancy checks (typescript-eslint/typescript-eslint#10744) (typescript-eslint/typescript-eslint#11812) - chore: update pkg

This PR contains the following updates: | Package | Change | Age | Confidence | |---|---|---|---| | [@typescript-eslint/eslint-plugin](https://typescript-eslint.io/packages/eslint-plugin) ([source](https://github.com/typescript-eslint/typescript-eslint/tree/HEAD/packages/eslint-plugin)) | [`8.47.0` -> `8.48.1`](https://renovatebot.com/diffs/npm/@typescript-eslint%2feslint-plugin/8.47.0/8.48.1) | [![age](https://developer.mend.io/api/mc/badges/age/npm/@typescript-eslint%2feslint-plugin/8.48.1?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://developer.mend.io/api/mc/badges/confidence/npm/@typescript-eslint%2feslint-plugin/8.47.0/8.48.1?slim=true)](https://docs.renovatebot.com/merge-confidence/) | | [@typescript-eslint/parser](https://typescript-eslint.io/packages/parser) ([source](https://github.com/typescript-eslint/typescript-eslint/tree/HEAD/packages/parser)) | [`8.47.0` -> `8.48.1`](https://renovatebot.com/diffs/npm/@typescript-eslint%2fparser/8.47.0/8.48.1) | [![age](https://developer.mend.io/api/mc/badges/age/npm/@typescript-eslint%2fparser/8.48.1?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://developer.mend.io/api/mc/badges/confidence/npm/@typescript-eslint%2fparser/8.47.0/8.48.1?slim=true)](https://docs.renovatebot.com/merge-confidence/) | --- ### Release Notes <details> <summary>typescript-eslint/typescript-eslint (@typescript-eslint/eslint-plugin)</summary> ### [`v8.48.1`](https://github.com/typescript-eslint/typescript-eslint/blob/HEAD/packages/eslint-plugin/CHANGELOG.md#8481-2025-12-02) [Compare Source](typescript-eslint/typescript-eslint@v8.48.0...v8.48.1) ##### 🩹 Fixes - **eslint-plugin:** \[restrict-template-expressions] check base types in allow list ([#11764](typescript-eslint/typescript-eslint#11764), [#11759](typescript-eslint/typescript-eslint#11759)) - **eslint-plugin:** honor ignored base types on generic classes ([#11767](typescript-eslint/typescript-eslint#11767)) - **eslint-plugin:** \[consistent-type-exports] check value flag before resolving alias ([#11769](typescript-eslint/typescript-eslint#11769)) ##### ❤️ Thank You - Josh Goldberg - OleksandraKordonets - SangheeSon [@Higangssh](https://github.com/Higangssh) - tao You can read about our [versioning strategy](https://typescript-eslint.io/users/versioning) and [releases](https://typescript-eslint.io/users/releases) on our website. ### [`v8.48.0`](https://github.com/typescript-eslint/typescript-eslint/blob/HEAD/packages/eslint-plugin/CHANGELOG.md#8480-2025-11-24) [Compare Source](typescript-eslint/typescript-eslint@v8.47.0...v8.48.0) ##### 🚀 Features - **eslint-plugin:** \[no-redundant-type-constituents] use assignability checking for redundancy checks ([#10744](typescript-eslint/typescript-eslint#10744)) ##### 🩹 Fixes - **typescript-estree:** disallow binding patterns in parameter properties ([#11760](typescript-eslint/typescript-eslint#11760)) - **eslint-plugin:** \[consistent-generic-constructors] ignore when constructor is typed array ([#10477](typescript-eslint/typescript-eslint#10477)) ##### ❤️ Thank You - Dima Barabash [@dbarabashh](https://github.com/dbarabashh) - JamesHenry [@JamesHenry](https://github.com/JamesHenry) - Josh Goldberg - mdm317 [@gen-ip-1](https://github.com/gen-ip-1) You can read about our [versioning strategy](https://typescript-eslint.io/users/versioning) and [releases](https://typescript-eslint.io/users/releases) on our website. </details> <details> <summary>typescript-eslint/typescript-eslint (@typescript-eslint/parser)</summary> ### [`v8.48.1`](https://github.com/typescript-eslint/typescript-eslint/blob/HEAD/packages/parser/CHANGELOG.md#8481-2025-12-02) [Compare Source](typescript-eslint/typescript-eslint@v8.48.0...v8.48.1) This was a version bump only for parser to align it with other projects, there were no code changes. 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mdm317 added 2 commits January 30, 2025 01:05

feat : work for object type in union types

e9c19a6

test : add test for object type in union types

6207864

mdm317 and others added 7 commits January 30, 2025 23:08

test : add more case

ac85ecb

feat : intersection type exists within a union type

11cc737

feat : intersection case

98ac978

fix : using getText instead of typetostring

f4939b7

we can not know typename {a : 2 } using typetostring type B = {a : 2}; type T = { a: 1 } | B

refactor

a6c822a

fix : Emit an error for a union type only when the objects have the s…

91b9cc5

…ame keys.

test: add test for union

81264c0

mdm317 added 7 commits March 4, 2025 01:56

feat : intersection case where objects have different keys

686e5a0

reafactor

6fa4e1b

fix : lint err

567e062

fix: improve test coverage

a81a06e

fix : lint err

7a8d574

Merge branch 'main' into 7742-no-redundant-type-constituents

ba0dd2d

fix : infinite loop at no unused var

70fbe0c

mdm317 commented Mar 5, 2025

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mdm317 commented Mar 5, 2025

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mdm317 marked this pull request as ready for review March 5, 2025 19:03

JoshuaKGoldberg requested changes Mar 17, 2025

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JoshuaKGoldberg added the awaiting response Issues waiting for a reply from the OP or another party label Mar 17, 2025

mdm317 added 2 commits March 30, 2025 17:25

refactor : use typeToString instead of getText

e2ded16

refactor: integrate message IDs and error reports function

2b9d3b0

mdm317 added 8 commits November 5, 2025 15:22

Merge branch 'main' into 7742-no-redundant-type-constituents

a554902

revert: spec file changes

91c1aa6

fix: lint

4e81ab4

fix: refrence error logic

034c243

refactor

d835eda

feat: handle array objecttype

c4bb7b4

fix: max call stack

3daae15

fix: test

acb39d8

mdm317 commented Nov 9, 2025

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mdm317 marked this pull request as ready for review November 9, 2025 08:51

JoshuaKGoldberg self-requested a review November 10, 2025 15:25

JoshuaKGoldberg removed the awaiting response Issues waiting for a reply from the OP or another party label Nov 10, 2025

JoshuaKGoldberg approved these changes Nov 18, 2025

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JoshuaKGoldberg added the 1 approval >=1 team member has approved this PR; we're now leaving it open for more reviews before we merge label Nov 18, 2025

bradzacher approved these changes Nov 23, 2025

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bradzacher merged commit 2ffb168 into typescript-eslint:main Nov 23, 2025
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chrisbbreuer mentioned this pull request Nov 24, 2025

chore(deps): update all non-major dependencies stacksjs/eslint-plugin#67

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timtucker-dte mentioned this pull request Nov 24, 2025

Bug: [no-redundant-type-constituents] Crashes with stack overflow for redundant uses of recursive array types #11787

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silverwind mentioned this pull request Dec 1, 2025

Bug: [no-redundant-type-constituents] False-positive with Record intersections #11811

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JoshuaKGoldberg mentioned this pull request Dec 1, 2025

fix(eslint-plugin): revert "[no-redundant-type-constituents] use assignability checking for redundancy checks" #11812

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feat(eslint-plugin): [no-redundant-type-constituents] use assignability checking for redundancy checks #10744

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