Discriminated unions in arrays have ambiguous member types | TypeScript Community | Page 1

keen axleBOT May 10, 2023, 2:39 PM

#

@junior wagon Here's a shortened URL of your playground link! You can remove the full link from your message.

RyanCaoDev#0428

Playground Link

Preview:```ts
enum I { A, B };

interface T<S extends I> { i: S; j: S extends I.A ? number : string; }

const t = [{
i: I.A,
j: "string also works",
}] satisfies T<I>[];```

agile ivy May 10, 2023, 2:43 PM

#

T.j is linked to S, not linked to T.i

woven hemlock May 10, 2023, 2:56 PM

#

also that's not a discriminated union (it's not a union type at all). this is:

keen axleBOT May 10, 2023, 2:56 PM

#

mkantor#7432

Playground Link

Preview:ts ... type T = { i: I.A; j: number; } | { i: I.B; j: string; } ...

charred igloo May 10, 2023, 3:10 PM

#

i think he meant union because S is treated as the union of all enum values and causes extends to always be true
can also be solved with https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#distributive-conditional-types

Create types which act like if statements in the type system.

keen axleBOT May 10, 2023, 3:10 PM

#

Jakob#8686

Playground Link

Preview:```ts
enum I { A, B };

type T<S extends I> = S extends any ? { i: S; j: S extends I.A ? number : string; } : never;

const t = [{
i: I.A,
j: "string also works",
}] satisfies T<I>[];

const u = [{
i: I.A,
j: 3
}] satisfies T<I>[];```

#Discriminated unions in arrays have ambiguous member types