Immer modifies `readonly` properties of nested instances that are not immerable

🐛 Bug Report

immer allows modification of readonly properties of nested class instances that are not marked as immerable. This seems dangerous to me. Just like immer does not allow direct modification of class instances that are not marked as immerable, an exception should be thrown. Even better, the types should not allow this.

import { immerable, produce } from "immer";

class Address {
  constructor(public readonly streetName: string) {
    this.streetName = streetName;
  }
}

class ImmerableAddress {
  [immerable] = true;

  constructor(public readonly streetName: string) {
    this.streetName = streetName;
  }
}

class ImmerablePerson {
  [immerable] = true;

  constructor(public readonly address: Address) {
    this.address = address;
  }
}

describe("immer", () => {
  const immerableAddress = new ImmerableAddress("foo");
  it("works fine with immerable nested class instance", () => {
    const produced = produce(new ImmerablePerson(immerableAddress), (draft) => {
      draft.address.streetName = "bar";
    });
    expect(produced.address.streetName).toBe("bar");
    expect(produced.address).toBeInstanceOf(ImmerableAddress);
    expect(produced.address).not.toBe(immerableAddress);
  });

  const address = new Address("foo");
  it("allows illegal modification of non-immerable nested class instance", () => {
    const produced = produce(new ImmerablePerson(address), (draft) => {
      draft.address.streetName = "bar"; // works unexpectedly
    });
    expect(produced.address.streetName).toBe("bar");
    expect(produced.address).toBeInstanceOf(Address);
    expect(produced.address).toBe(address);
  });
});

Link to repro

CodeSandbox demo

Environment

Immer 10.1.1

Start with the linked CodeSandbox reproduction and inspect the existing handling of non-immerable class instances and readonly typing. Done means the nested Address case is rejected consistently, with regression coverage for the runtime and/or type behavior; the issue does not choose between those outcomes.