Calling return() on a suspendedStart iterator helper doesn't seem to call return() on the underlying

[syg]

Iterator helpers are specified as generators whose bodies are Abstract Closures. That is, they start in a suspendedStart state until one of its step methods is called. If you call return() on a helper before next(), this goes to GeneratorResumeAbrupt, which has a fast path for going from suspendedStart -> completed without evaluating anything. AFAICT this won't call return() on the underlying, which is the intention.

[bakkot]

Agreed that this is a bug.

I think this can be fixed by adding a special case for suspendedStart in %IteratorHelperPrototype%.return ( ), though it will require moving the iterated record from a captured alias to a slot on the object. (Which is annoying, because the objects are created by CreateIteratorFromClosure, which doesn't currently allow you to specify additional slots on the created object.)