Bug: a instanceof b === false doesn't influence type flow

[jeengbe]

Bug Report

🔎 Search Terms

instanceof explicit false if type narrowing

🕗 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
• I was able to reproduce this on versions 5.1.0-dev.20230304

⏯ Playground Link

Playground link with relevant code

💻 Code

declare const x: unknown;

() => {
    if(x instanceof Error === false) return;

       x;
    // ^?
    // Should be 'Error', is 'unknown'
}

() => {
    if(!(x instanceof Error)) return;

       x;
    // ^?
    // Is correctly 'Error'
}

🙁 Actual behavior

The semantically equivalent expressions (x instanceof Error === false) and !(x instanceof Error) influence type flow (correct term?) differently.

🙂 Expected behavior

(x instanceof Error === false) should function the same way !(x instanceof Error) does.

[jcalz]

Duplicate of #31105 and #9508 (is this a bug or a won’t fix?)

[jeengbe]

a won't fix, I wasn't aware of this limitation