When for-in is invoked directly on a proxy, then it simply executes the iterator returned by the proxy handler. That prescribes a much more rigid iteration sequence than for regular objects that breaks existing implementations. In particular, implementations often want to compute the list of keys before the iteration starts, which would currently be forbidden due to observable calls to iterator.next.

This is inconsistent not just with regular objects, but also with enumeration over regular objects that have a proxy as their prototype -- because in that case the object's [[Enumerate]] still has the liberty to invoke the prototype's [[Enumerate]] whenever.

To fix this, we probably need to loosen the spec for proxy [[Enumerate]], such that it does not prescribe that the original iterator is invoked in lock-step.