bug#37888: 27.0.50; Streams and errors in element generation

[Noam Postavsky]

Michael Heerdegen <michael_heerdegen@web.de> writes:
>
> Would something like this make sense (Noam)?

>     ((eq (car-safe stream) stream--fresh-identifier)
> -    (setf (car stream) stream--evald-identifier)
> -    (setf (cdr stream) (funcall (cdr stream))))
> +    (prog1 (setf (cdr stream) (funcall (cdr stream)))
> +      (setf (car stream) stream--evald-identifier)))

Right, only mark the stream element as evaluated after we (successfully!)
evaluate it.  Makes sense to me.