TypeScript 5.5 "Control Flow Narrowing for Constant Indexed Accesses" is not working

[Alexsey]

🔎 Search Terms

5.5 Control Flow Narrowing for Constant Indexed Accesses

🕗 Version & Regression Information

• This is a crash - No
• This changed in commit
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.1-rc#code/C4TwDgpgBA0lC8UDkAPJUA+yRIFC9EigHkEoBvXKaqFALigDsBXAWwCMIAnXAX3wAmEAMYAbAIZdowgPaMAzsCgBrBjFxCxk6XMVQZDYvgCWAMygAKZVGON9ASgpUashTNEQAdKJkBzCzIA2soAuvZ8QA

💻 Code

type K = 'x' | 'y'

type O = {
 x: number
}

declare const k: K
declare const o: O

if (k in o) {
 // Element implicitly has an 'any' type because expression of type 'K' can't be used to index type 'O'.
 // Property 'y' does not exist on type 'O'.(7053)
 console.log(o[k])
}

🙁 Actual behavior

The error is present in the v5.5.1-rc. The error text:

Element implicitly has an 'any' type because expression of type 'K' can't be used to index type 'O'. Property 'y' does not exist on type 'O'.(7053)

🙂 Expected behavior

The error should've been resolved in the TypeScript 5.5

Additional information about the issue

No response

[MartinJohns]

What makes you think this should work? Neither the commit nor the blog mentions the in operator.

[Alexsey]

Martin Johns (@MartinJohns) you are correct, I mistakingly assumed a broader feature scope. Anyway, considering this logic TypeScript cannot guess a better type for O[k] then unknown