Type of a generic is not narrowed

[valya-smoryakova]

Bug Report

🔎 Search Terms

discriminated type union, narrowing, generics

🕗 Version & Regression Information

• This changed between versions 3.3.3 and 3.5.1

⏯ Playground Link

Playground link with relevant code

💻 Code

type A = {
    s: string;
    n: number;
};

// K here may be 's'|'n' or 's' or 'n' (but not never as I understand, as f cannot accept argument of type never in runtime)
function f<K extends keyof A>(key: K): A[K] {
    const k1: typeof key = 's';    // It is completely fine for this line to have an error (as it is outside if check and K may be 'n')

    if (key === 's') {
        // In fact we know that type of x === 's' here.
        // Thus return type here should be A['s'] in other words it should be a string
        // But TypeScript still thinks it is of type A[K]
        
        const k2: typeof key = 's';   // does not work. Here I expect typeof key to be 's'
        const c: A[K]   = 'test';     // does not work. Here I expect A[k] to be string
        const d: A['s'] = 'test';     // works
        
        return 'test';                // does not work. Here I expect 'test' to be asignable to A[K]
    }
    
    if(key === 'n') {
        return 10;                    // the same
    }

    throw new Error();
}

🙁 Actual behavior

Typescript ignores the check on the variable typed with a subtype of union type and is unable to narrow type

🙂 Expected behavior

Typescript narrows the type

My guess is that this behavior may somehow be related to https://devblogs.microsoft.com/typescript/announcing-typescript-3-5/#fixes-to-unsound-writes-to-indexed-access-types

[gerich-home]

Looks similar to #24085

[jcalz]

Or #33014

[RyanCavanaugh]

#33014 is the best fit for this. Type parameter themselves are not currently narrowable the way the OP implies they should be

[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.