Type parameter leak caused by `this` and reverse mapped type

Description

declare function test<T extends Record<string, unknown>>(obj: {
  [K in keyof T]: () => T[K];
}): T;

const obj = test({
  //   ^? const obj: { a: number; b: T[string]; }
  a() {
    return 0;
  },
  b() {
    return this.a();
  },
});

Contributor guide

Tech stack

typescript

Domain

tooling

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.

fresh

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

TypeScript type systemMapped typesTypeScript compiler internals

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

5

Research direction

The bug reproduces in the TypeScript playground with the provided code. The issue is that the type parameter T leaks into the result type when using reverse mapped types with `this`. The fix likely involves the type checker's handling of reverse mapped types and `this` in `src/compiler/checker.ts`. There are no linked pull requests, so this is fresh work.