Microsoft/TypeScript

Distributed keyof over union leads to erroneous indexed access

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#49,000 opened on May 6, 2022

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 (11 comments) (1 reaction) (0 assignees)TypeScript (6,726 forks)batch import
BugDomain: Indexed Access TypesHelp Wanted

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Description

Bug Report

🕗 Version & Regression Information

This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

type KeyOf<T> = T extends unknown ? keyof T : never

type Wrong<T> = { [K in KeyOf<T>]: T[K] }

🙁 Actual behavior

It is accepted by the type checker, but it shouldn't be allowed because T could be a union of objects with unrelated properties. The computed type doesn't even have any sense:

type test= Wrong<{ prop0: string } | { prop1: number; prop2: boolean } | { prop3: string[] }>
//   { prop0: unknown; prop1: unknown; prop2: unknown; prop3: unknown; }

🙂 Expected behavior

K shouldn't be allowed to index type T.

Contributor guide

Research direction
Investigate the type checker's handling of distributed keyof and indexed access types in mapped types.
Tech stack
typescript
Domain
tooling
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.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScriptDistributive conditional types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20