IsUnion: Fix behavior when the entire union extends all individual members#1353
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sindresorhus merged 5 commits intosindresorhus:mainfrom Feb 7, 2026
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som-sm
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Feb 7, 2026
test-d/is-union.ts
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| type SuperTypeUnion = | ||
| {optionalField?: number; test1: number; test2: string} | ||
| | {test1: number; test2: string}; | ||
| expectType<true>({} as IsUnion<SuperTypeUnion>); |
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| type SuperTypeUnion = | |
| {optionalField?: number; test1: number; test2: string} | |
| | {test1: number; test2: string}; | |
| expectType<true>({} as IsUnion<SuperTypeUnion>); | |
| type TestUnion = | |
| // Here, the entire union extends both the individual members: | |
| // So, `TestUnion` extends `{opt?: number; a: number; b: string}`, and | |
| // `TestUnion` extends `{a: number; b: string}`. | |
| | {opt?: number; a: number; b: string} | |
| | {a: number; b: string}; | |
| expectType<IsUnion<TestUnion>>(true); |
This makes the test case more clear.
source/is-union.d.ts
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| ? false | ||
| : T extends any | ||
| ? [U] extends [T] | ||
| ? true extends IsEqual<[U], [T]> |
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| ? true extends IsEqual<[U], [T]> | |
| ? IsEqual<U, T> extends true |
U and T don't need to wrapped in a tuple anymore.
IsUnionIsUnion: Fix behavior when the entire union extends all individual members
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Thanks for your quick reviews! Addressed! |
som-sm
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Feb 7, 2026
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Close #1351