Microsoft/TypeScript

Problem with function types, <T>() => T extends X ? () : ()

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#56,721 opened on Dec 9, 2023

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 (10 comments) (0 reactions) (0 assignees)TypeScript (6,726 forks)batch import
Cursed?Domain: check: Variance RelationshipsHelp WantedPossible Improvement

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Description

🔎 Search Terms

"function", "generic function"

🕗 Version & Regression Information

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAYgdgHgBoD4oF4oICooBQCUGa2UEAHsBHACYDOUSUA-FAIxQBcUATAFChIUAKIBHAK4BDADZ02yADRQAmmnR8osRKjKVq9DVoSqWUYACdx0bgDMZdCHwHhoAdXMB7OAHMAShDpxaWAMEQl7eQBvcwhJGi9pEChJbgByAEFUgF8lSJSoDOyUJydBaHh5HUwcfCJ0El0qWgYmVg5ufjKtHmQ1LFxCYihSCib6RlN23mchMSlZHqQlE3VNCt7G-TpDeB6TVgsrLig7WUcZ6ABhD3MYgGNgf0Dg0Ln7HujY+LhE5LTMnJQPL-IpAA

💻 Code

type Fn<X> = <T>() => T extends X ? 1 : 2
type Equals<X, Y> =
  Fn<X> extends
  Fn<Y> ? true : false

type A = Equals<{readonly a: 'A'}, {a: 'A'}>

🙁 Actual behavior

type A should shows true.

🙂 Expected behavior

type A should be false.

It works well when the code is like this.

type Fn1<X> = <T>() => T extends X ? 1 : 2
type Fn2<X> = <T>() => T extends X ? 1 : 2
type Equals<X, Y> =
  Fn1<X> extends
  Fn2<Y> ? true : false

type A = Equals<{readonly a: 'A'}, {a: 'A'}>

Additional information about the issue

No response

Contributor guide

Research direction
Examine the implementation of the Equals type in the TypeScript compiler and trace how conditional types with generics are resolved.
Tech stack
typescript
Domain
None
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 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
TypeScriptGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25