Inconsistent behaviour of -?

[m-shaka]

🔎 Search Terms

optional mapped type

🕗 Version & Regression Information

• This changed between versions 5.4.5 and 5.5.4

⏯ Playground Link

https://www.typescriptlang.org/play/?exactOptionalPropertyTypes=true#code/C4TwDgpgBA8mwEsD2A7AhgGwCpIKouRQHUFgALfAEwgDMEUJKAeLAPigF4oBvAKCgFQA2gGko9KAGsIIJDShYAugC4FUCAA9gEFJQDOUAEoQAxkgBOzEQBoFoxa36DnAfjsjFT5wNVZ7UAB8oAFddWnpGXgBfXl5QSCgASRRtcwBbBEoENG1OWHhCTBx8QhJyKnCGZm4oNBdVPWBzegBzKCj2AHpOnlr6qEbmlDag0Oo6Kva48GgAGVIIc0w8mrqGptbAkLCJxnaoboG0NOh46DQDZNSMrJyIWLOjCD0ARjzjAEdghHNGJivFjdstouj1Vushm0Yo9jHoAEzvCBfH5-eapTCg3poCGbUY7CKUdpAA

💻 Code

type OptionalToUnionWithUndefined<T> = {
    [K in keyof T]: T extends Record<K, T[K]>
        ? T[K]
        : T[K] | undefined
}

type Intermidiate = OptionalToUnionWithUndefined<{ a?: string }> // { a?: string | undefined }
type Literal = { a?: string | undefined } // same type as Intermidiate

type Res1 = Required<Intermidiate> // { a: string }
type Res2 = Required<Literal> // { a: string | undefined }

🙁 Actual behavior

Res1 and Res2 are different types

🙂 Expected behavior

they should be the same because the inputs are

Additional information about the issue

This issue may share the cause with #59902

[SmolPatat]

I think TS is tripping balls and hallucinating here. Both Res1 and Res2 are supposed to be {a: string | undefined}, and they are!
The following code compiles:

type Res1a = Res1['a']; // string | undefined
let r: Res1 = {a: undefined}; // Ok

type Id<T extends object> = {[K in (keyof T & string)]: T[K]};
type Res1Id = Id<Res1>; // { a: string | undefined }

This issue might be related to the --exactOptionalPropertyTypes flag. However, Required works differently without the flag, so I am not certain.

[RyanCavanaugh]

There's no -? in the code sample; what's the relation to the title of the issue?

[SmolPatat]

There's no -? in the code sample; what's the relation to the title of the issue?

The -? is in Required. Hope this helps.

[RyanCavanaugh]

I think this is just a type display issue. If you actually access the property, it has the expected type:

type OptionalToUnionWithUndefined<T> = {
    [K in keyof T]: T extends Record<K, T[K]>
        ? T[K]
        : T[K] | undefined
}

type Intermidiate = OptionalToUnionWithUndefined<{ a?: string }> // { a?: string | undefined }
type M = { a?: string }
type Literal = { a?: string | undefined } // same type as Intermidiate

type Res1 = Required<Intermidiate> // { a: string }
type Res2 = Required<Literal> // { a: string | undefined }

declare let r: Res1;
const j = r.a;
// j: string | undefined

// OK
const p: Res2['a'] = undefined;