function's union generic parameter cannot be infered from condition

[zaaack]

TypeScript Version: 2.7.2

Search Terms:

Code

type Props = {
    a: string
    b: number
}

function test<K extends keyof Props>(key: K, newVal: Props[K], oldVal: Props[K]) {
    if (key === 'a') { // K won't be inferred as 'a'
        shouldBeString(newVal)  // and this line would trigger an error
    } else if (key === 'b') {
        shouldBeNumber(newVal)
    }
}

function shouldBeString(v: string) {

}


function shouldBeNumber(v: string) {

}

Expected behavior:

Generic union should be infered.

Actual behavior:

Not

Playground Link: link
Related Issues:

[RyanCavanaugh]

We don't currently have the ability to track relations from one type guard to another symbol. See also #12184

[zaaack]

@RyanCavanaugh Thanks for your fast reply! I see.