Michael Ficarra: in Iterator.prototype.flatMap https://tc39.es/ecma262/multipage/control-abstraction-objects.html#sec-iterator.prototype.flatmap

step 6.b.viii.4.b.ii: IfAbruptCloseIterator(backupCompletion, iterated).

afaict it is impossible for backupCompletion not to be abrupt here? previous step produces it as a result of IteratorClose(innerIterator, completion), and IteratorClose can only result in a non-abrupt completion if passed a non-abrupt completion to begin with