runtime: Generator/AsyncGenerator prototype method descriptors + brand checks

[proggeramlug]

Summary

%GeneratorPrototype% and %AsyncGeneratorPrototype% are missing their method
property descriptors and method brand checks. A fresh parity sweep (2026-06-02,
after #3664 wired the generator intrinsics) shows 53 runtime failures still
concentrated here — the prototype methods layer wasn't covered by #3664.

Evidence

Descriptor conformance on the prototype methods (next/return/throw/
constructor) — should be configurable:true:

• GeneratorPrototype/prop-desc.js, AsyncGeneratorPrototype/prop-desc.js
  — "next/return/throw descriptor should be configurable"
• AsyncGeneratorPrototype/{constructor,length,name}.js
  — "… descriptor should be configurable"

Method brand check / dispatch:

• AsyncGeneratorPrototype/this-val-not-async-generator.js
  — TypeError: then is not a function (should throw a TypeError brand error
  when next/return/throw is called with a non-async-generator this)

(Plus a few async-driver edge cases: "Detected unsettled top-level await",
"request-queue-order" — likely a separate async-generator resumption gap.)

Repro

function* g() {}
const proto = Object.getPrototypeOf(g.prototype);
console.log(Object.getOwnPropertyDescriptor(proto, 'next').configurable); // expect true

const GenProto = Object.getPrototypeOf(Object.getPrototypeOf((function*(){})()));
GenProto.next.call({});   // expect TypeError (this is not a generator)

Suggested scope

Install the generator/async-generator prototype methods with spec-correct
descriptors and add the this-brand check to next/return/throw. The
descriptor half overlaps the builtin-descriptor issue; the brand-check half
overlaps the arg-validation work (#3146) but is specific to the generator
prototypes.

---

Filed from the parity-gap radar, 2026-06-02 re-sweep. Cluster: 53
Generator/AsyncGeneratorPrototype runtime failures. Follow-on to closed #3664.

[andrewtdiz]

I am taking this as part of a focused runtime descriptor/prototype-method cut with #4139. Intended PR scope: install Generator/AsyncGenerator prototype method descriptors with Node-compatible flags, add receiver brand checks for next/return/throw where the method exists, and cover the current reflection regressions with focused parity tests. Non-goals: async-generator scheduling/order edge cases and the active TypedArray constructor PR #4143.

[andrewtdiz]

Opened draft PR #4149 for the related generator descriptor slice:

• adds parity coverage for GeneratorPrototype/AsyncGeneratorPrototype method descriptors and bad-receiver behavior
• fixes generator/async-generator constructor toStringTag reflection through own Symbol.toStringTag
• keeps the existing generator-intrinsics parity fixture passing under Node 25

Validation includes the Node 25 descriptor parity filter, generator-intrinsics smoke, cargo build, fmt, diff, and file-size checks.

[andrewtdiz]

Updated draft PR #4149 with the rebased descriptor implementation at commit 2ed014f. The refreshed cut keeps Generator/AsyncGenerator prototype descriptor coverage in the new globals fixture, includes own Symbol.toStringTag reflection for generator constructors, and has current local validation: release build, both descriptor parity filters, cargo runtime object tests, fmt, diff checks, and file-size check.