Loss of type inference after assignment

[zkrising]

Bug Report

🔎 Search Terms

Loss of type information, Forgotten type inference.

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about common bugs that aren't bugs.

⏯ Playground Link

Playground link with relevant code

💻 Code

let x: unknown = "foo";

if (typeof x === "string") {
    // typescript now knows that this is a string from inference.
    let y: string = x;

    // perform any assignment to x;
    // I found this by calling x = x.substring(1), but it appears just reassigning x works.
    x = x;

    // similarly; x = "bar" still results in x being unknown. Any assignment to x results in the type inference being forgotten.

    // x is now unknown?
    // Type 'unknown' is not assignable to type 'string'.(2322)
    y = x;
}

🙁 Actual behavior

Typescript correctly infers that x is a string if typeof x === "string", however, it seems to forget this if x is assigned to, including trivial assignments.

🙂 Expected behavior

x is inferred to be a string, so x = x should be a no-op -> x should still be a string. This issue in and of itself is not too important, but any assigment to x (including ones that guaranteeably return a string, like x = x.substring(1) also causes TS to forget its type inference.)

I apologise if this is known or not a bug, I could not find anything similar reported on the issue tracker.

[zkrising]

More information: This seems to only happen with x: unknown. This does not happen with other cases of type inference, such as when x: string | number, or even x: any. This is presumably a bug with unknown.

[RyanCavanaugh]

See #27706