Microsoft/TypeScript

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

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#57,109 opened on Jan 20, 2024

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 (1 comment) (2 reactions) (0 assignees)TypeScript (6,726 forks)batch import
BugDomain: Indexed Access TypesHelp Wanted

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Description

🔎 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

Contributor guide

Research direction
Investigate the TypeScript compiler's handling of conditional types with literal types and keyof. Trace the code path that produces error 2538 when a literal type is used as an index type after checking with 'extends keyof'. Look at the checker's getIndexedAccessType or related functions.
Tech stack
typescript
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScriptType system fundamentals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30