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IsEqual: Fix generic inference #1306

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Merged
sindresorhus merged 3 commits into from
Dec 2, 2025
Merged

IsEqual: Fix generic inference #1306

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1ad7a25
Fix: Correct type inference for generic component props using `IsEqual`
taiyakihitotsu Nov 30, 2025
17f7a13
update test
taiyakihitotsu Dec 1, 2025
be6fa79
refactor: delete empty line
taiyakihitotsu Dec 1, 2025
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12 changes: 4 additions & 8 deletions source/is-equal.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -26,14 +26,10 @@ type Includes<Value extends readonly any[], Item> =
@category Utilities
*/
export type 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>
[A] extends [B]
? [B] extends [A]
? _IsEqual<A, B>
: false
: false;

// This version fails the `equalWrappedTupleIntersectionToBeNeverAndNeverExpanded` test in `test-d/is-equal.ts`.
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8 changes: 8 additions & 0 deletions test-d/is-equal.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -85,3 +85,11 @@ expectType<true>(equalTupleIntersectionToBeNeverAndNeverExpanded);

declare const equalTupleIntersectionAndTuple: IsEqual<[{a: 1}] & [{a: 1}], [{a: 1}]>; // eslint-disable-line @typescript-eslint/no-duplicate-type-constituents
expectType<true>(equalTupleIntersectionAndTuple);

// Test for Issue https://github.com/sindresorhus/type-fest/issues/1305
type FieldProps<T1> = {
onChange: <T2>(value: T2, shouldBeTrue: IsEqual<T1, T2>) => void;
};
const _TextField = <T>(value: T) => {
({} as FieldProps<T>).onChange(value, true); // eslint-disable-line @typescript-eslint/consistent-type-assertions
};
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@som-sm som-sm Dec 1, 2025

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Use the following test instead:

Suggested change
// Test for Issue https://github.com/sindresorhus/type-fest/issues/1305
type FieldProps<T1> = {
onChange: <T2>(value: T2, shouldBeTrue: IsEqual<T1, T2>) => void;
};
const _TextField = <T>(value: T) => {
({} as FieldProps<T>).onChange(value, true); // eslint-disable-line @typescript-eslint/consistent-type-assertions
};
type Assignability<T, U, _V extends IsEqual<T, U>> = any;
type TestAssignability<T> = Assignability<T, T, true>;

This is easier to understand and aligns with other generic assignability tests that we have.

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@taiyakihitotsu taiyakihitotsu Dec 1, 2025

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Thanks, addressed.