Type-asserted variable become unknown again at once change

[fenying]

Bug Report

A type-asserted variable from unknown, would become unknown again at once its value changed

🔎 Search Terms

🕗 Version & Regression Information

• This problem could be reproduced between versions 4.3.5 (earlier versions not tested) and latest nightly

⏯ Playground Link

Playground link with relevant code

💻 Code

function isNumber(s: unknown): s is number { // it's the same even changed to string or other types

    return typeof s === 'number';
}

let c: unknown = 123;

if (!isNumber(c)) {

    throw new TypeError('what?');
}

c = c + 1;

/**
 * Why would an asserted number become unknown again?
 */
if (c + 1 === 0) {

    console.log(c);
}

🙁 Actual behavior

the assert number-typed variable c became unknown again.

🙂 Expected behavior

It should keep being number type

[MartinJohns]

Duplicate of #27706. Used search terms: unknown narrow in:title