Missing error about inability to access conflicting private properties on generic types involving intersections

Description

class A {
  private a: { foo: number };
}

class B {
  private a: { bar: string };
}

type X<T extends A> = [T["a"], (T | B)["a"]]; // errors

type Y<T extends A | B> = T["a"]; // error

type Z<T extends A & B> = T["a"]; // no error?

type Z_<T extends A & B> = T["a"]["b"]["c"]["d"]; // look, we can even go crazy and access deep unknown properties

type Z1<T extends A> = T["a"]; // error

type Z2<T extends B> = T["a"]; // error

type Z3<T extends A, T2 extends B> = (T & T2)["a"]; // no error?

type Z3_<T extends A, T2 extends B> = (T & T2)["a"]["b"]["c"]["d"]; // look, we can even go crazy and access deep unknown properties

Contributor guide

Tech stack

typescript

Domain

developer experiencetooling

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.

1-2 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 genericsTypeScript intersection typesTypeScript private members

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

15

Research direction

Investigate how TypeScript resolves private property accessibility on intersection types (e.g., T & B). The bug is that errors are not reported for generic types constrained to intersections (e.g., T extends A & B) when accessing private properties. Look at PR #37762 which removed the error; the old test (diff ca0ed2db9207146d34393eb3428bf91872b53a7d882b1e010312ec6f02b9d08aL24) shows the expected behavior. Reproduce with the playground link and compare behavior for types like Z and Z3. The fix likely involves ensuring the accessibility check in the checker respects intersections of generic type parameters.