Literal tested as key of a certain type cannot be used as an index type

🔎 Search Terms

"literal", "extends", "keyof", "cannot be used to index type", "conditional"

🕗 Version & Regression Information

Versions: ^5.0 (regression)

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.3&ssl=7&ssc=54&pln=1&pc=1#code/C4TwDgpgBAKlC8UDeBYAUFKBrAXFAziALYBGA9gDYA06mZwAFhAE57mUQCGAdlVAPT8oPek2bYIIAhAhF8UZhACOAVwCWigCZQAZmXGNoJFQHMowMsLCROzdAF906QbHDQA5ACIsn91ADGPNz0UCTQKvgQ2pzyPFBq3JoQAB7mbgB0ABQATACsAMwAHACU6KCQUADCCFDuWH4pwBCJ8liSZDqwUAD8sADade4AulB43BAAbiwA3AJCPCBAA

💻 Code

type T = {
  k: symbol,
  other: boolean, // another key seems required for the bug to appear
}

// Type '"k"' cannot be used as an index type.(2538)
type C = 'k' extends keyof T ? T['k'] : never; // any

🙁 Actual behavior

An error is produced in the positive clause of the conditional type.
The resulting type is incorrect.

🙂 Expected behavior

No error.

Additional information about the issue

No response

Start by running the TypeScript Playground reproduction linked in the issue with TypeScript 5.3.3, then compare the behavior across the reported ^5.0 versions. Trace the conditional type and indexed-access checking involved in the example; done means the positive clause produces no error and the resulting type is correct.