Narrowing on obj.prop doesn't propagate to obj

[kevinjwz]

🔎 Search Terms

narrowing intersection property type guard assertion

🕗 Version & Regression Information

occurs on:
5.3.3
5.4.0-dev.20240204

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.0-dev.20240204#code/JYOwLgpgTgZghgYwgAgILIN4Chm+QDwH4AuZEAVwFsAjaAbiwF8tRJZEUAhZCfSEACYBnNJhx58pCjXpMsWGORAIwwAPYhkMABRxSqAJRi8yYDGTawATwAOENebgA6fMgC8H5ACJptKF4NsExNyIQgAOSo-XRcDOmQAegTkNQBrcWDkUIhOXTjE5OgoNSgM5GZmBSUVdU1s3OpSTiNsOSrlVQ0ssMiZKG1JMijoQPlKpNNwaDCaroA3adqsaztkAGF3YwkSIb6GZhWUABFNjYAyLdxB31lKxQ7arQBmbQRSNZayswtDh2QEFzuTw+Yb+UaZXDZXrRAH4fITNJlEJhI6veGFKDFUomCrye6zOoo7QCUhHT5yIA

💻 Code

interface A {
    x?: number;
}
interface B extends A {
    x: number;
}

function f(a: A) {
    if (typeof a.x === "number"){
        useNumber(a.x); // ok
        useB(a); // error
    }
}

function useB(b: B) {}

function useNumber(x: number) {}

🙁 Actual behavior

Cannot assign a to parameter of type B. It seems type narrowing works independently on a.x and a.

🙂 Expected behavior

a should be assignable to parameter of type B.

Additional information about the issue

No response

[MartinJohns]

Duplicate of #42384 and many many others.

[fatcerberus]

oops @MartinJohns beat me to it this time

[typescript-bot]

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