Can I get "narrowing to never" behavior on a non-union type?

[cakoose]

Bug Report

🔎 Search Terms

narrow non-union never
narrow record never

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "narrow" and "never".

(I tested on the Playground with v3.3.3333, v4.0.5, v4.6.0-dev.20211201.)

⏯ Playground Link

Playground link with relevant code

💻 Code

export function impossible(_value: never): never {
    throw new Error('impossible! (according to static types)');
}

// This is how we define errors for some of our operations.
export type Operation1Error =
    | {type: 'not_found'}
    | {type: 'not_authorized', details: string};

// We use this pattern to handle errors. If someone adds a new option to
// Operation1Error, TS will tell us we need another branch.
function f1(err: Operation1Error): void {
    if (err.type === 'not_found') {
        // ...
    } else if (err.type === 'not_authorized') {
        // ...
    } else {
        throw impossible(err);
    }
}

// Some of our operations currently have just a single type of error.
export type Operation2Error =
    | {type: 'not_found'}

// This doesn't work as well -- TS doesn't allow us to use the same pattern.
function f2(err: Operation2Error): void {
    if (err.type === 'not_found') {
        // ...
    } else {
        // TS error: Argument of type '{ type: "not_found"; }' is not assignable to parameter of type 'never'.
        throw impossible(err);
    }
}

🙁 Actual behavior

In f2, I get a type error on impossible(err). (See comment in code sample.)

🙂 Expected behavior

In f2, I was hoping that err gets narrowed to never and I don't get a type error when using impossible(err).

[MartinJohns]

Duplicate of #38963.

[RyanCavanaugh]

If you have something that is currently 1 case but will eventually be more than 1, you can use a workaround

export type Operation2Error =
    | {type: 'not_found'}
    | {type: never}

[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.