Microsoft/TypeScript

Index type for an intersected `NoInfer` can get sometimes incorrectly deferred

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#61,091 opened on Jan 31, 2025

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Domain: Index TypesHelp WantedPossible Improvement

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Description

🔎 Search Terms

noinfer intersection index type keyof deferred deferral

🕗 Version & Regression Information

  • This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20250131#code/C4TwDgpgBAKhDOwoF4oGsIgPYDMoAoA5LASQDscIAnAHgG8ocssAuKRKgSzIHMoBfAHxQAZFDr8AlAG4AUAHp5UZQD0A-LNCRYCYACYU6TLgIMmrdsC68Bo8VLmLlUdbKA

💻 Code

type Test = keyof (NoInfer<{ foo: string }> & {});
//   ^? type Test = keyof (NoInfer<{ foo: string; }> & {})
type Test2 = keyof ({ foo: string } & {});
//   ^? type Test2 = "foo"

🙁 Actual behavior

Test is a deferred index type

🙂 Expected behavior

Test shouldn't be a deferred index type but a "foo" literal, just like Test2.

Additional information about the issue

No response

Contributor guide

Research direction
Investigate why keyof (NoInfer<{ foo: string }> & {}) is deferred. Trace through the type instantiation logic in the checker, focusing on how NoInfer interacts with intersection types and keyof.
Tech stack
typescript
Domain
developer experiencetooling
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.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScriptcompiler codebase
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40