fix(eslint-plugin): [no-unnecessary-condition] use assignability checks in checkTypePredicates by zaewc · Pull Request #12147 · typescript-eslint/typescript-eslint

zaewc · 2026-03-20T08:22:17Z

PR Checklist

Addresses an existing open issue: fixes Enhancement: [no-unnecessary-condition] use assignability checks in checkTypePredicates #11798
That issue was marked as accepting prs
Steps in Contributing were taken

Overview

checkTypePredicates previously only flagged a type guard call as unnecessary when the argument's
type was the exact same TS type object as the predicate's asserted type. This meant cases
where the argument is a strict subtype of the asserted type were silently passed over.

e.g, isStringOrNumber(s) where s: string was not flagged, even though string is always
assignable to string | number, making the check redundant.

This pr extends the check to also report when the argument type is a strict subtype of the asserted
type — i.e., assignable one-way but not mutually assignable. Mutual assignability is explicitly
excluded to avoid false positives with structurally compatible types that still produce meaningful
narrowing.

typescript-eslint · 2026-03-20T08:22:22Z

Thanks for the PR, @976520!

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codecov · 2026-03-20T08:27:34Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.71%. Comparing base (b3315fd) to head (1496319).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12147   +/-   ##
=======================================
  Coverage   86.71%   86.71%           
=======================================
  Files         513      513           
  Lines       16270    16271    +1     
  Branches     5059     5060    +1     
=======================================
+ Hits        14108    14109    +1     
  Misses       1474     1474           
  Partials      688      688

Flag	Coverage Δ
unittest	`86.71% <100.00%> (+<0.01%)`	⬆️

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

Files with missing lines	Coverage Δ
.../eslint-plugin/src/rules/class-methods-use-this.ts	`97.87% <ø> (ø)`
...slint-plugin/src/rules/no-unnecessary-condition.ts	`97.98% <100.00%> (+<0.01%)`	⬆️
...plugin/src/rules/no-unnecessary-type-parameters.ts	`92.81% <ø> (ø)`
...-plugin/src/rules/no-useless-default-assignment.ts	`91.54% <ø> (ø)`
packages/typescript-estree/src/check-modifiers.ts	`10.47% <ø> (ø)`

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…ks in checkTypePredicates

… assignability in checkTypePredicates

… from subtype check

bradzacher

looking good so far

interface Wider {
  a: string;
}
interface Narrower {
  a: string;
  b?: number;
}
declare function isWider(x: unknown): x is Wider;
declare const n: Narrower;
if (isWider(n)) {
}

Could you add this as a test case please.
It should error IIUC

…on-subtype-check

zaewc · 2026-04-08T10:40:05Z

@bradzacher

The rule wasn't catching this because it compared types with ===, so structurally compatible but distinct interfaces were missed. Fixed by switching to checker.isTypeAssignableTo(). This also uncovered that string literal -> string type guard cases (e.g. isString('falafel')) were incorrectly allowed, moved those to invalid as well.

…types and literals

Simplify the type guard check to use `checker.isTypeAssignableTo()` directly instead of restricting to union types. This catches cases where the argument type is structurally assignable to the predicate type (e.g. interfaces with optional properties, string literals).

Use mutual assignability and union subtype checks instead of reference equality to detect unnecessary type guards. This catches cases like interfaces with optional properties (e.g. Narrower assignable to Wider) and subtypes of union predicate types (e.g. string subtype of string | number). Add eslint-disable for false positives caused by custom TypeScript typings that make structurally different runtime types appear equivalent.

bradzacher

thanks!

JoshuaKGoldberg

🆒 !

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ArnaudBarre · 2026-04-19T15:35:35Z

I think the last test case should be allowed, it's useful to have valid access to the b property:

https://www.typescriptlang.org/play/?#code/JYOwLgpgTgZghgYwgAgOrACbWQbwFDLJwBcyAzmFKAOYDceAvnqJLIigHJxRQD2A7tnyES5SjXqEARgH5SIAK4BbKdHpMsCADbcUMBSARhgvEMmBkuPAdAAUAD1IGA1iAEgAlKXvmyyK3yCUPSaOlAoCKYUyPyk6FjBePwAdFLMMMi2FgE2ULb8Hh64BDGpjHhAA

bradzacher · 2026-04-19T21:31:40Z

@ArnaudBarre you don't need a predicate at all in the last test case because b is an optional property. So you don't need a runtime assertion at all.

Eg const n: Narrower = w is valid TS code when the "new" property is marked as optional.

If the property is non-optional then the check is required and the rule does not report.

ArnaudBarre · 2026-04-19T21:57:13Z

Oh yeah true, that also explain the edge case (even if I think it would be better implemented by actually checking the the narrower type has at least one required property)

The fact that you need to create a new variable in was not be obvious to me and could be added in the message

zaewc added 3 commits March 21, 2026 13:40

fix(eslint-plugin): [no-unnecessary-condition] use assignability chec…

270750b

…ks in checkTypePredicates

test(eslint-plugin): [no-unnecessary-condition] add tests for subtype…

6bd88eb

… assignability in checkTypePredicates

fix(eslint-plugin): suppress no-unnecessary-condition false positives…

fb9a6d4

… from subtype check

zaewc force-pushed the fix/no-unnecessary-condition-subtype-check branch from 14c521b to fb9a6d4 Compare March 21, 2026 04:43

fix(eslint-plugin): clarify comment

926c1f1

bradzacher changed the title ~~fix(eslint-plugin): use assignability checks in checkTypePredicates~~ fix(eslint-plugin): [no-unnecessary-condition] use assignability checks in checkTypePredicates Apr 8, 2026

bradzacher changed the title ~~fix(eslint-plugin): [no-unnecessary-condition] use assignability checks in checkTypePredicates~~ fix(eslint-plugin): [no-unnecessary-condition] use assignability checks in checkTypePredicates Apr 8, 2026

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bradzacher added the awaiting response Issues waiting for a reply from the OP or another party label Apr 8, 2026

Merge branch 'typescript-eslint:main' into fix/no-unnecessary-conditi…

7bd6531

…on-subtype-check

zaewc added 2 commits April 8, 2026 19:45

fix(eslint-plugin): detect unnecessary type guards for assignable sub…

17cafe3

…types and literals

zaewc requested a review from bradzacher April 8, 2026 11:02

github-actions bot removed the awaiting response Issues waiting for a reply from the OP or another party label Apr 8, 2026

zaewc force-pushed the fix/no-unnecessary-condition-subtype-check branch from 2a1a1f9 to 9f79dd4 Compare April 8, 2026 11:49

zaewc force-pushed the fix/no-unnecessary-condition-subtype-check branch from 9f79dd4 to a07a056 Compare April 8, 2026 12:03

zaewc and others added 2 commits April 9, 2026 08:32

Merge branch 'main' into fix/no-unnecessary-condition-subtype-check

8b49ff6

Add passing test for interface subtype narrowing

1496319

bradzacher added the 1 approval >=1 team member has approved this PR; we're now leaving it open for more reviews before we merge label Apr 9, 2026

bradzacher approved these changes Apr 9, 2026

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JoshuaKGoldberg approved these changes Apr 11, 2026

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bradzacher merged commit 7c9e06f into typescript-eslint:main Apr 12, 2026
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ci-lockfile-regen bot mentioned this pull request Apr 17, 2026

chore(dev-deps): bump the dev-minor-patch group across 1 directory with 4 updates ably/ably-cli#350

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chore(deps): update dependency typescript-eslint to v8.58.2 lacolaco/blog.lacolaco.net#1515

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fix(eslint-plugin): [no-unnecessary-condition] use assignability checks in checkTypePredicates#12147

fix(eslint-plugin): [no-unnecessary-condition] use assignability checks in checkTypePredicates#12147
bradzacher merged 10 commits intotypescript-eslint:mainfrom
zaewc:fix/no-unnecessary-condition-subtype-check

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