Asserts on functions defined using function statements do not work

[pawbor]

Bug Report

🔎 Search Terms

asserts function declaration

⏯ Playground Link

Playground link with relevant code

💻 Code

function fn<T>(val: T) {
    return { val }
}

const fnLambda = function <T>(val: T) { return { val } };

const arrow = <T,>(val: T) => ({ val });

type AnyFunction = (...args: never[]) => unknown;

function enhance<Fn extends AnyFunction>(fn: Fn): asserts fn is Fn & { enhanced: true } {
    Object.assign(fn, { enhanced: true } as const);
}

(function runner1() {
    enhance(fn);
    enhance(fnLambda);
    enhance(arrow);

    console.log(fn.enhanced)
    console.log(fnLambda.enhanced)
    console.log(arrow.enhanced)
}())

🙁 Actual behavior

Assertion does not work for function declaration but it works for function expressions / arrow functions.

🙂 Expected behavior

I expect same behavior in all 3 cases.

[MartinJohns]

Duplicate of #41232. Used search terms: asserts function in:title

[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.