Narrowing return type inside a generic function using object map

[alesmenzel]

Bug Report

Incorrectly narrowing type inside a generic function where the return type is indexed from an object.

🔎 Search Terms

Generics, Mapped types, index types

🕗 Version & Regression Information

• This is a crash No
• This changed between versions - on version 3.3.3333 did not error, but was not correct type either
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about generics, index types

⏯ Playground Link

Playground link with relevant code

💻 Code

type A = {
    a: number
}

type B = {
    b?: string
}

type MAP = {
    a: A,
    b: B
}

function getProfilesForNetwork<Network extends keyof MAP>(network: Network): MAP[Network] {
    if (network === 'a') {
        return { a: 43 }
    }
    if (network === 'b') {
        return { b: 'bbbbb' }
        // ERROR - Property 'a' is missing in type '{ b: string; }' but required in type 'A'.(2322)
        // Network is generic over 'a' | 'b' so it can be only 1 of those values, the value should narrow the return type to B here
    }
    throw new Error('Unknown!')
}

🙁 Actual behavior

TS expects the return type to be A & B.

🙂 Expected behavior

The network === 'a' / network === 'b' should narrow the return type and should be either A or B.

[RyanCavanaugh]

This isn't doing what you think it's doing; there isn't narrowing on type parameters like the code sample implies:

function getProfilesForNetwork<Network extends keyof MAP>(network: Network): MAP[Network] {
    if (network === 'a') {
        return { a: 43 }
    }
    if (network === 'b') {
        return { a: 32 } // <- no error
    }
    throw new Error('Unknown!')
}

See #33014

[typescript-bot]

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