TypeScript can't infer correct type from while loop if while condition is given a promise

[BlackAsLight]

🔎 Search Terms

promise in while loop condition

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Bugs that aren't bugs

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.2.0-dev.20230807#code/GYVwdgxgLglg9mABAZwDYFN0AcAUBbZALkTBDwCN0AnASmIAUq48Zl0AeKKkdAPkQDeAKESIq6KCCpIw6AO6JGzVhy49eOAIaIAvPzZQAKjDzo4IKDhw1d-TTjXoaAGkQEaNIQF8hQzcgBPSERQSFgERHQAD008LAwARmsGJhY2dmQuGDAAc35hAEgMKEQAN01UHmJMqmycxAAfRHAAE3RgbPQWoQK5AAsYDEQHbidBHoLNOU0YErRMXABWAAZVzwKC8sr0XUQAckW9ifJxTQBrHp8C8UlpMoqeb19-IIgQ8Gh4JGjY+PQAJmSilSKgyWVy+R6xXu22q4PqTVa7U63V6AyGWmmsxQGGwOBWaxshU2Dx2On2h2OpwuBSuNykSC2jy8QA

💻 Code

function sleep(ms: number): Promise<true> {
  return new Promise<true>(a => setTimeout(() => a(true), ms))
}

async function example1(): Promise<string> {
	let value: string | undefined
	while (true) {
		await sleep(5000)
		value = '5'
		break
	}
	return value
}

async function example2(): Promise<string> {
	let value: string | undefined
	while (await sleep(5000)) {
		value = '5'
		break
	}
	return value
}

🙁 Actual behavior

In example2, TypeScript can't infer the type of value and that it should be now a string and not undefined simply because the Promise that returns true in the while loop has been moved to the while condition instead of the body of the while loop.

🙂 Expected behavior

The expected behaviour should be that TypeScript should be able to infer that it is still a string at the return statement just like in example1

[jcalz]

Duplicate of #30551 possibly?

#29323 then

[fatcerberus]

await is a red herring here - if (true) vs. if (trueTypedValue) are treated differently by CFA, iirc

[fatcerberus]

Explanation: #29323 (comment)

Basically, it's a design limitation and unlikely to be fixed easily. CF nodes are created conservatively, before types are known. Things like if (true) and if (false) work only because those are special-cased syntactically.

I do question why you would write this code - if you know the condition is always going be true, why make it conditional in the first place?

[RyanCavanaugh]

I'm not 100% sure I buy my previous explanation as applies to this case. The CFA nodes are there and we can resolve the type without circularity in this case when assessing reachability.

That said, it's obviously not a super useful fix in terms of addressing "real" code.

[BlackAsLight]

I do question why you would write this code - if you know the condition is always going be true, why make it conditional in the first place?

I like to put the sleep command in the while condition to simply write less code. I always make my sleep command return true in the event I'd want to use it in some condition.

If for example though I wanted a do-while loop and the body of that loop had several conditions with some calling continue, I wouldn't want to put the sleep function just above all those continue keywords.