A contextually-typed signature coming from correlated union can't be assigned back to source of its contextual type

Description

type Definitions = {
  onFoo: [arg: number];
  onBar: [arg: string];
};

type SomeCallbacks = {
  [K in keyof Definitions]: (...args: Definitions[K]) => void;
};

const wrapCallback = <K extends keyof SomeCallbacks>(
  source: SomeCallbacks,
  target: SomeCallbacks,
  key: K,
) => {
  const callback = source[key];

  target[key] = (...args) => {
    if (Math.random() > 0.5) {
      return callback(...args);
    }
  };
};

function wrapAll(callbacks: SomeCallbacks): SomeCallbacks {
  const wrapped = {} as SomeCallbacks;
  for (let key in callbacks) {
    wrapCallback(callbacks, wrapped, key as keyof SomeCallbacks);
  }
  return wrapped;
}

Contributor guide

Tech stack

typescript

Domain

developer experience

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

5

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

over 1 week

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.

needs investigation

Prerequisites

TypeScript type systemcorrelated unionsmapped types

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

15

Research direction

The issue demonstrates that a contextually typed signature from a correlated union is not assignable back to the source of its contextual type. This is a type checking bug in the TypeScript compiler. To investigate, start by examining the assignability logic in the checker.ts file, specifically how contextual types interact with union types and mapped types. Look for existing tests related to correlated unions to understand expected behavior. The fix likely requires adjusting the type assignment rules to preserve the contextual type information when assigning functions with rest parameters.