Microsoft/TypeScript

Incorrect type inference for generic class in JavaScript

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#61,552 opened on Apr 5, 2025

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BugDomain: check: Type InferenceHelp Wanted

Description

Does this issue occur when all extensions are disabled?: Yes

  • VS Code Version: 1.99.0
  • OS Version: Windows 10 Home 22H2

Steps to Reproduce:

  1. Define a generic class as below: 
    /** @template T */
class C {
  /** @param {T} a */
  constructor(a) { }
  f() { const v = new C(/** @type {T[]} */ ([])); }
}
  2. The type of the variable v is incorrectly inferred to be C<T[][]>.

If the constructor call is outside of the class C<T> (as in the example below), the return type of the call is correctly inferred to be C<T[]>.

/** @template T */
class D {
  f() { const v = new C(/** @type {T[]} */ ([])); }
}
/** @template T; @param {C<T>} c */
const f = c => { const v = new C(/** @type {T[]} */ ([])); }

Contributor guide

Tech stack
javascripttypescript
Domain
toolingdeveloper experience
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.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScriptJavaScriptgeneric classesJSDoc type annotations
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Investigate the TypeScript type checker (likely in src/compiler/checker.ts) to understand how generic types are inferred when constructing a generic class inside its own method. The issue indicates that the type variable T is incorrectly wrapped when the constructor call occurs within the class C's method. Compare with behavior outside the class to identify the discrepancy in generic type resolution.