Per-evaluation class: #private access inside a generator method skips the evaluation-brand check #11185

Summary

For a class expression evaluated more than once (a per-evaluation class), a private access inside a generator method doesn't enforce the per-evaluation brand. Calling evaluation A's generator method on an instance of evaluation B reads B's private field, where the spec requires a TypeError. Non-generator methods reject the call correctly. This applies to string-keyed and [Symbol.iterator] generator methods alike.

Repro (no packages)

function mk() {
  return class {
    #v: number;
    constructor(v: number) { this.#v = v; }
    *gen() { yield this.#v; }
    get() { return this.#v; }
  };
}
const A = mk(), B = mk();
function t(l: string, f: () => any) { try { console.log(l, f()); } catch (e: any) { console.log(l, "threw", e.constructor.name); } }
t("gen cross", () => (A.prototype as any).gen.call(new B(9)).next().value);
t("method cross", () => (A.prototype as any).get.call(new B(9)));

• Node 26.5.1: gen cross threw TypeError / method cross threw TypeError
• Perry origin/main c7d0963 (perry-dev, PERRY_NO_AUTO_OPTIMIZE=1, Linux x86_64): gen cross 9 / method cross threw TypeError

Likely cause

js_private_guard compares the receiver's evaluation brand with the brand of the lexical this (brand_owner), or with the PRIVATE_LEXICAL_BRAND_STACK top. A generator body runs later, inside the next closure, so neither the method's lexical class evaluation nor its receiver-independent brand is available there. The guard then compares the receiver with itself and passes. The generator needs to capture the evaluation brand at generator creation, or the brand-stack entry needs to be re-established around each next/return/throw.

Found while fixing #11170. Possibly adjacent to the per-evaluation class work in #11141 / #11142.