Tagged union inference in mapped type with generic key

[felipeochoa]

TypeScript Version: 2.6.2; Playground

Code

interface T {
    a: { k: 'n', v: number },
    b: { k: 's', v: string },
}

function f<K extends 'a'|'b'>(t: T[K]) {
    if (t.k === 'n') {
        return t.v * 5;
    } else {
        return t.v.charCodeAt(0);
    }
}

Expected behavior: t should be narrowed so that the two clauses are accepted by the compiler

Actual behavior: t.v is typed as string | number in both branches.

This is similar to #17930, but this issue adds the generic K to the mix.

[DanielRosenwasser]

As solved in #17930, I don't think you need K in this example.

interface T {
    a: { k: 'n', v: number },
    b: { k: 's', v: string },
}

function f(t: T[keyof T]) {
    if (t.k === 'n') {
        return t.v * 5;
    }
    else {
        return t.v.charCodeAt(0);
    }
}

Do you have specific pattern you're trying to model?

[felipeochoa]

I realize K is superfluous here, but this was an illustrative example. In the real case, I'm using mapped types to narrow multiple values simultaneously. The real function is more like:

function f<K extends 'a'|'b'>(k: K, t1: T1[K], t2: T2[K])

which I type that way to ensure T1 and T2 are compatible (i.e. correspond to the same union case).

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.