typescript passes `number | Promise<number>` as if it were `number` (or something like that)

Bug Report

Something is very weird that makes typescript pass Promise<number> as if it were a number when it is an object field. Typescript correctly recognizes that Promise<number> is not a number when it is a plain variable. Weird.

🔎 Search Terms

promise numeric

🕗 Version & Regression Information

I believe this has always existed in typescript. I have tried multiple versions on playground (from 3.3 up to 5). I did read the FAQs in https://github.com/Microsoft/TypeScript/wiki/FAQ#common-bugs-that-arent-bugs and found nothing related.

⏯ Playground Link

Playground link with relevant code

💻 Code

const n: number | Promise<number> = Promise.resolve(0);
console.log(n >= 0);  // Correctly rejected: Operator >= cannot be applied to Promise<number> and number.

const foo: {n: number | Promise<number>} = {
  n: Promise.resolve(0),
};
console.log(foo.n >= 0); // Incorrectly type checks.  Runtime result is bogus.

🙁 Actual behavior

The last expression passes the type checker even though it is clearly wrong.

🙂 Expected behavior

That the last expression is rejected with Operator >= cannot be applied to Promise<number> and number..

[Andarist]

I’ve checked what happens under the hood and it seems that it’s enough for left and right types to be comparable in either direction: https://github.dev/microsoft/TypeScript/blob/c7f49bceed878d751701f018d066fffc5926e3b0/src/compiler/checker.ts#L35641

This is quite surprising but apparently it’s even OK to compare class instances (test case taken from the TS test suite): TS playground

I spiked a potential improvement for the reported case here but I’m not totally sure if this is something that the TS team would like to incorporate.

[azizghuloum]

Thanks for explaining the difference @fatcerberus. (though it’s also weird that it narrows despite my explicit type specification but let’s not get distracted)

Now why number | Promise<number> typechecks is still the issue.