Argument of type [type] is not assignable to parameter of type [typeof itself]

[lionel-rowe]

🔎 Search Terms

Type narrowing, typeof, type casting, as, argument of type is not assignable to parameter of type typeof itself

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about typeof, type narrowing

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAGlC8UDeUDWBLAdgEwFxQHIBDAqAH0ICNSBfAKDuwgGMAbIgJ2gDMBXTZsHQB7TFG6YAFJXwoMOfAWpQaASnwA3YemwMmbTtGaiAzsCgAPfDAbpuUSRYB087AniIlBVcjpR-4lIWqn4BEo5QRCZQoJDC9sGh-uEWkdGxEPGWUABkyGhYeDHgmQkuhSoh9EA

💻 Code

type X = { kind: 'a' | 'b' }

declare function fn(b: { kind: 'b' }): void

declare const x: X

if (x.kind === 'b') {
    // Argument of type 'X' is not assignable to parameter of type '{ kind: "b"; }'.
    fn(x)
    // Argument of type 'X' is not assignable to parameter of type '{ kind: "b"; }'.
    fn(x as typeof x)
    // ok
    fn(x as typeof x & { kind: typeof x.kind })
}

🙁 Actual behavior

First 2 versions give type errors.

🙂 Expected behavior

All 3 versions pass type checking.

Additional information about the issue

It seems like all the type info is available and inferred correctly by TS, given that the third version works, which makes it even weirder that the first two fail. How are x and x as typeof x different from x as typeof x & { kind: typeof x.kind }?

[jcalz]

Duplicate of #42384

[lionel-rowe]

Duplicate of #42384

@jcalz Thanks! I was sure someone would have raised it before but searching threw up nothing relevant. Closing as dupe.