Microsoft/TypeScript

Empty types and types with `never` as a key behave differently

Open

#56,691 opened on Dec 6, 2023

View on GitHub
 (6 comments) (2 reactions) (0 assignees)TypeScript (6,726 forks)batch import
BugDomain: Mapped TypesHelp Wanted

Repository metrics

Stars
 (48,455 stars)
PR merge metrics
 (Avg merge 6d 17h) (9 merged PRs in 30d)

Description

🔎 Search Terms

Mapped type unknown

🕗 Version & Regression Information

  • This changed between versions 4.7.4 and 4.8.4

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.0-dev.20231206#code/CYUwxgNghgTiAEYD2A7AzgF3gVwFw5QGsUkB3FAbgCgqMBPABwQEF4BeeAbwG0BpeAJYp4KEADcQMALr4oKOgF8qAemXx1APQD8NVfABySeAFEYMJDCrJ0WKPhTYIEeAB8CoAGZCQwV1wXsONR66vDaumqm5pbWmPAARvaOzm7YKJ7evm6sHNjBaqHhQA

💻 Code

declare const u: unknown;

type A = {[K in never]: any}
//   ^?

// No Error
const a: null | undefined | {} = u;
//    ^?

// Error
const b: null | undefined | A = u;
//    ^?

🙁 Actual behavior

The types a and b appear to be the same, but only the assignment to b causes an error.

🙂 Expected behavior

I'm not sure which is better, I think it's either no error or both errors.

Additional information about the issue

Both were errors in version 4.7.4

Contributor guide

Research direction
Investigate the difference in behavior between empty object type `{}` and a mapped type with `never` keys in type assignment. Examine the TypeScript type checker to understand why `A` is not assignable from `unknown` while `{}` is. Check for changes between versions 4.7.4 and 4.8.4 that might have caused this regression.
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.
Half day
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.
Needs investigation
Prerequisites
TypeScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40