Computed generic argument type is not narrowed

[qwelias]

Bug Report

🔎 Search Terms

computed generic function argument guard narrow

🕗 Version & Regression Information

v4.3.2

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about generics

⏯ Playground Link

Playground link with relevant code

💻 Code

type Select<T extends 'a' | 'b'> = {
    a: string;
    b: number;
}[T];

const select = <T extends 'a' | 'b'>(type: T, data: Select<T>) => {
    if (type === 'b') return data * 5; // The left-hand side of an arithmetic operation must be of type 'any', 'number', 'bigint' or an enum type.(2362)
    return 1;
}

🙁 Actual behavior

Cannot data * 5 because The left-hand side of an arithmetic operation must be of type 'any', 'number', 'bigint' or an enum type.(2362)

🙂 Expected behavior

data * 5 is valid because it's within the code branch where type === 'b' and type of data in that case is number

[qwelias]

There's a workaround to use tuple type for arguments

type SelectArgs =
    | ['a', string]
    | ['b', number]

const select = (...args: SelectArgs) => {
    if (args[0] === 'b') return args[1] * 5;
    return 1;
}

But in that case one cannot destructure them, because then it would break for some reason, so have to keep using indexes :(
I also tried to overload function signature, but it had similar issue IIRC

[MartinJohns]

Because such an operation would be unsound:

select<'a' | 'b'>('b', 'hello')

You would need #27808 first.

---

You have the very common misconception that T extends 'a' | 'b' means T can either be 'a' or 'b'. But T can be 'a' | 'b' as well.

[qwelias]

So is the only way right now to do something like this is to use tuples like in #44358 (comment) (or analogous object type) without the ability to destructure?
ts pg

[MartinJohns]

Yes. Destructure would require #12184 / #44169.

[qwelias]

Thanks for links, looks like it depends on existing issues.