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IsEqual: Fix behaviour when instantiated with never and unknown #1251

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Merged
sindresorhus merged 3 commits into from
Sep 28, 2025
+12 −2
Merged

IsEqual: Fix behaviour when instantiated with never and unknown #1251

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6ae93a0
`is-equal`: Fix unexpected true result for never and unknown
taiyakihitotsu Sep 28, 2025
ec81f87
`is-equal`: Fix unknown and never case
taiyakihitotsu Sep 28, 2025
863ffe7
refactor: lowerCase to upperCases
taiyakihitotsu Sep 28, 2025
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10 changes: 8 additions & 2 deletions source/is-equal.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -26,8 +26,14 @@ type Includes<Value extends readonly any[], Item> =
@category Utilities
*/
export type IsEqual<A, B> =
[A, B] extends [infer A, infer B]
? _IsEqual<A, B>
[A, B] extends [infer AA, infer BB]
? [AA] extends [never]
? [BB] extends [never]
? true
: false
: [BB] extends [never]
? false
: _IsEqual<AA, BB>
: false;

// This version fails the `equalWrappedTupleIntersectionToBeNeverAndNeverExpanded` test in `test-d/is-equal.ts`.
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4 changes: 4 additions & 0 deletions test-d/is-equal.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,10 @@ expectType<true>({} as IsEqual<never, never>);
expectType<false>({} as IsEqual<any, never>);
expectType<false>({} as IsEqual<never, any>);
expectType<false>({} as IsEqual<any, unknown>);
expectType<false>({} as IsEqual<never, unknown>);
expectType<false>({} as IsEqual<unknown, never>);
expectType<false>({} as IsEqual<[never], [unknown]>);
expectType<false>({} as IsEqual<[unknown], [never]>);
expectType<false>({} as IsEqual<[any], [never]>);
expectType<true>({} as IsEqual<[any], [any]>);
expectType<true>({} as IsEqual<[never], [never]>);
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