Closed
Description
🔎 Search Terms
"exactOptionalPropertyTypes", "strict type equality"
🕗 Version & Regression Information
- Tested in TypeScript 5.8.2
⏯ Playground Link
💻 Code
// exactOptionalPropertyTypes:
type T1 = {
prop?: string,
}
type T2 = {
prop?: undefined | string,
}
type _ = [
// ❌ T1 and T2 are considered strictly equal
Assert<Equal<
T1, T2
>>,
// T2 is not assignable to T1
Assert<Not<Assignable<
T1, T2
>>>,
// T1 is assignable to T2
Assert<Assignable<
T2, T1
>>,
// T1 and T2 are not considered loosely equal
Assert<Not<LooseEqual<
T1, T2
>>>,
];
type Assert<_T extends true> = void;
// 🟢 Trick to trigger strict type equality. Source: https://github.com/Microsoft/TypeScript/issues/27024#issuecomment-421529650
type Equal<X, Y> =
(<T>() => T extends X ? 1 : 2) extends
(<T>() => T extends Y ? 1 : 2) ? true : false
;
type LooseEqual<X, Y> =
[X] extends [Y]
? ([Y] extends [X] ? true : false)
: false
;
type Not<B extends boolean> =
// `Equal` because want to avoid Not<any> being `false` or `true`
Equal<B, true> extends true
? false
: (Equal<B, false> extends true ? true : never)
;
type Assignable<Target, Source> = [Source] extends [Target] ? true : false;🙁 Actual behavior
T1 and T2 are considered to be strictly equal – even though T2 is not assignable to T1.
🙂 Expected behavior
T1 and T2 should not be considered strictly equal.
Additional information about the issue
- Small repository that demonstrates the bug: https://github.com/rauschma/exactOptionalPropertyTypes
- Related issue elsewhere: false equality with exactOptionalPropertyTypes mmkal/expect-type#128
- Related Zod bug: .optional() and --exactOptionalPropertyTypes colinhacks/zod#635