Wrong local variable type when assigned in lambda. #61313

🔎 Search Terms

type, infer, lambda, callback

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.3#code/GYVwdgxgLglg9mABAgogJzXNAKAFgQzABMAbAUzQC5Fszr1M0BKRAXgD5EA3OGIlgN4BYAFCJxiAsXI4wZAO6IGWbEyYBuUQF9Ro8lEQVGlZWgA+YECRJtEl65pGjUGFWTadhYiUay2yjloauiIwwLSuaIgAhKysdlYkgqISiFC4mIq+aIGiQA

💻 Code

function onError(handler: (e: Error) => void) {
    handler(new Error());
}

let error:Error|null = null;

onError(e => {
    error = e;
});

if(error !== null) {
    throw error;
}

🙁 Actual behavior

The type of error is inferred as null in line 11 and unknown in line 12. This is wrong because there is a potential assignment to error` in the local lambda.

🙂 Expected behavior

The type of error should be Error|null after the onError call as there is a potential assignment happening in the callback. Its understandable if the analyzer cannot widen the type if the passed callback is again some other function or method but for local lamdbas it could be treated like a "conditional" which might be met. e.g. in the following example error is widened to Error|null

function onError(handler: (e: Error) => void) {
    handler(new Error());
}

let error:Error|null = null;

if(Math.random() > 0.5) {
    error = new Error();
}

if(error !== null) {
    throw error;
}

Additional information about the issue

One workaround I currently have is to add error = error as Error | null; somewhere.