Microsoft/TypeScript

Non-exported recursive type alias is inlined until it falls back to any in declarations

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#58,696 opened on May 29, 2024

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BugDomain: Declaration EmitHelp Wanted

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Description

🔎 Search Terms

declarations recursive types

🕗 Version & Regression Information

  • This is the behavior in every version I tried (before 4.4 it was either a stack overflow crash or an error on the recursive type)

⏯ Playground Link

Playground Link

💻 Code

// @declaration: true
// @filename: utils.ts 

type Recursive<T>  =  T extends Array<infer V>? Recursive<V>: T;

export const f = <T>(): Recursive<T> => { return null! } 

// @filename: index.ts 
import { f } from './utils';
export const a = f;

🙁 Actual behavior

In declarations, the type for a is printed as a: () => T extends (infer V)[] ? V extends (infer V)[] ? V extends (infer V)[] ? V extends (infer V)[] ? V extends (infer V)[] ? V extends (infer V)[] ? V extends (infer V)[] ? V extends (infer V)[] ? V extends (infer V)[] ? V extends (infer V)[] ? V extends (infer V)[] ? any : V : V : V : V : V : V : V : V : V : V : T;

🙂 Expected behavior

Recursive can't be represented without falling back to any (as seen above) so it should just be an error instead of silently to any

Additional information about the issue

Related to #55832

Contributor guide

Research direction
Examine the declaration emitter code in TypeScript's checker or emitter to understand how non exported recursive type aliases are inlined and eventually fall back to `any`. Focus on the logic that handles recursive type references and the decision to fall back. Look for relevant tests and the fix in related issue #55832.
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.
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.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40