fix: AsyncGeneratorComplete cannot call user code

[aapoalas]

It shouldn't be possible for a Call to both be asserted to never throw an error and call user code (except if the user function is known to be an async function I guess, but I don't think that's the case here?). I believe in this case the user-code effect is the wrong one.

[bakkot]

Unfortunately, it can! Consider

Object.defineProperty(Object.prototype, 'then', { get(){ console.log('user code'); } });

let gen = (async function* gen() {})();
gen.return();

Tracing through the execution, this will call AsyncGeneratorCompleteStep, and step 7.d will call promiseCapability.[[Resolve]] passing { value: undefined, done: true }, where promiseCapability is the promise capability created in %AsyncGeneratorPrototype%.return step 2. That means its [[Resolve]] is the function defined in Promise resolve function, and in step 9 of that algorithm you can see it does does a Get(resolution, "then"), where resolution is the { value: undefined, done: true } object created earlier. And since that object inherits from Object.prototype, doing a Get on its then will (in this case) trigger user code. However, it will wrap any errors from that user code in a rejected promise, so the Call itself will still complete normally.

The assumption that a Call which never throws must also not run user code usually holds, but not always, which is why we have an explicit marker for cases like this.

[aapoalas]

Ah, my bad then. Thank you for the explanation; trying to wrap one's mind around the async stuff is really hard :D