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IsUnion: Fix behavior when the entire union extends all individual members #1353

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Merged
sindresorhus merged 5 commits into from
Feb 7, 2026
Merged

IsUnion: Fix behavior when the entire union extends all individual members #1353

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7166b78
fix: #1351 `IsUnion`
taiyakihitotsu Feb 7, 2026
d4942f5
revert: remove meaningless changes
taiyakihitotsu Feb 7, 2026
55296dd
refactor: rename test name
taiyakihitotsu Feb 7, 2026
0d6fd7e
refactor: remove meaningless tuple, add a doc in tests
taiyakihitotsu Feb 7, 2026
e78d8e3
refactor: add a new line
taiyakihitotsu Feb 7, 2026
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3 changes: 2 additions & 1 deletion source/is-union.d.ts
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Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
import type {IsNever} from './is-never.d.ts';
import type {IsEqual} from './is-equal.d.ts';

/**
Returns a boolean for whether the given type is a union.
Expand All @@ -24,7 +25,7 @@ type InternalIsUnion<T, U = T> =
IsNever<T> extends true
? false
: T extends any
? [U] extends [T]
? IsEqual<U, T> extends true
? false
: true
: never
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9 changes: 9 additions & 0 deletions test-d/is-union.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -16,3 +16,12 @@ expectType<IsUnion<'foo' | 'bar'>>(true);
expectType<IsUnion<'foo' | 'bar' | 1>>(true);
expectType<IsUnion<'foo' | 1>>(true);
expectType<IsUnion<[] | {}>>(true);

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);