Bug Report

🔎 Search Terms

index type, indexed access type, index access operator, extends, Exclude, conditional type

🕗 Version & Regression Information

This is the behavior in every version I tried (3.3.3 to 4.3.2), and I reviewed the FAQ for entries about indexed access types.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface A {
    details1?: {y:string};
    details2?: {x:string};
}

function create<PropName extends keyof A>(
    name: PropName , a: A): Exclude<A[PropName], undefined> {
    const x: A[PropName] = a[name];
    if (x) return x; // Type error: Type 'A[PropName]' is not assignable to type 'Exclude<A[PropName], undefined>'.
    throw new Error();
}

🙁 Actual behavior

TypeScript complains that x in if (x) return x; is of type 'A[PropName]' and is not assignable to type 'Exclude<A[PropName], undefined>', because it assumes that x can be undefined.

🙂 Expected behavior

I would have expected that this works, as return x is the consequence of the conditional if (x), so x cannot be undefined.

Thanks for taking the time to look at this!