The union type cannot be determined using 'string []' to determine the type

[crazyair]

⏯ Playground Link

Playground link with relevant code

💻 Code

type Type1 = { string?: 'a'; array?: readonly ['a']; getType?: (data: 'a') => any };
type Type2 = { string?: 'b'; array?: readonly ['b']; getType?: (data: 'b') => any };

export const type1: Type1 | Type2 = { string: 'a', getType: data => data }; // "a"

export const type2: Type1 | Type2 = { array: ['a'] as const, getType: data => data }; // any

🙁 Actual behavior

input ['a'], getType data isany

🙂 Expected behavior

input ['a'], getType data isa

[crazyair]

I used const readonly, but still couldn't get it

[jcalz]

Please properly fill out the Version & Regression Information section.

Tuples aren't treated as discriminants; this is essentially a duplicate of #48500.

[crazyair]

Can we release the first level of derivation?

[crazyair]

Add keywords

[jcalz]

If I understand you correctly (and that's a big if, there seems to be a language barrier here), what you're asking for would require #18758

[jcalz]

"This changed between versions 4.9 and 5.1" <-- this means that it was working as you desired in version 4.9 and is broken in version 5.1. Is that really what you mean? I'm going to disengage now because it seems like there's a communication problem and I don't know how to fix it. Good luck.

[crazyair]

Can it only support (string | number)[]

[RyanCavanaugh]

A reference-typed discriminant is difficult to reason about and would break a lot of assumptions that CFA uses. These just aren't supported and are unlikely to be supported in the future.

[crazyair]

The initial requirement was

interface TableProps<T = any> {
  // dataIndex support `a | b | [a] | [b]`
  columns: { dataIndex: keyof T | [keyof T]; render: (value: T[keyof T]) => any }[];
}

export const data: TableProps<{ a: string; b: number }> = {
  columns: [
    { dataIndex: ['a'], render: value => value }, // render value type is string
    { dataIndex: ['b'], render: value => value }, // render value type is number
  ],
};

I don't know how to write the type either

[crazyair]

Can I convert ['a' , 'b'] to a-b

[typescript-bot]

This issue has been marked as "Question" and has seen no recent activity. It has been automatically closed for house-keeping purposes.