Intersection with a mapped type breaks bypassing private assignability.

Description

declare class X {
	private a: number;
	b: string;
}

type Y = { a: number } & { b: string };
declare let y: Y;
let x: X = y as Y & X;

type Z = Pick<{ a: number }, 'a'> & { b: string };
declare let z: Z;
let x: X = z as Z & X;

Contributor guide

Tech stack

typescript

Domain

backend

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.

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.

clear

Prerequisites

TypeScript fundamentalsPrivate class membersIntersection and mapped types

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

30

Research direction

Investigate why an intersection type created with a mapped type (Pick) fails to bypass private assignability while a direct object literal intersection does. Look at the TypeScript checker code in src/compiler/checker.ts, specifically the handling of intersection types and private property assignability (likely around the checkTypeRelatedTo or isRelatedTo functions). Examine how mapped types are resolved and how their structural compatibility differs from direct intersections. Also review existing tests for private class members and mapped types to understand the expected behavior.