Incorrect narrowing of boolean type when variable is mutated in a timer

[TAGraves]

🔎 Search Terms

"no-unnecessary-condition", "boolean", "constant", "mutated", "conditional", "timer"

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250318&ssl=9&ssc=1&pln=1&pc=1#code/KYDwDg9gTgLgBDAnmYcCSBnAoiAhgYxgB4AVAGjgFUA+OAXjgG0SBdOUGYAOwBMMnKbAPxwAFI0HsQnXv2bCEUAK6oAXHABmuADYZgASjjqtu4AG4AUKEixNSroQCWELnFwY9sEsmCkpMvkUVOAAfTR09alEAfXUSQwBvAF8LC21geAAjCAhtenDTSz0YEkcAW2AIJRhRUUM6WgS4bNz8mGVzOCSKAEZ9SwtcAHdcR3guYCG4AAUoCDLHPVEoYAxcgDdUBrhi0oqqmrr6WhW17U3Rex5gDUcJnn1egAYn-X7Bj2AvHyJMHAJiEgUBANM0ctoKCZIlEoQZLEA

💻 Code

export type IsExact<T, U> = [T] extends [U] ? ([U] extends [T] ? true : false) : false;
export function assertType<T extends true | false>(_: T) {}

let bool = false;
setTimeout(() => { bool = true; }, 1);

await new Promise(resolve => setTimeout(() => resolve(undefined), 100));
assertType<IsExact<typeof bool, false>>(false);

🙁 Actual behavior

Argument of type 'false' is not assignable to parameter of type 'true'. In this case, that means that bool is narrowed to false on line 8.

🙂 Expected behavior

The type of bool should be boolean because it will be true if the timer on line 5 ran and false if it did not. (In practice, with this exact code snippet, it is always true, so it definitely should not be narrowed to false!)

Additional information about the issue

No response

[MartinJohns]

Duplicate of #9998. There's even a dedicated issue template for this: https://github.com/microsoft/TypeScript/blob/main/.github/ISSUE_TEMPLATE/types-not-correct-in-with-callback.md?plain=1

[TAGraves]

Thank you -- I apologize for missing that!