Sequence destructuring compiled its right-hand side straight into
array_pad(), which throws "Argument #1 must be of type array" when a
Traversable is provided:
{% do [a, b] = items %}
with an IteratorAggregate/Generator for items crashed with a TypeError.
Coerce Traversables via iterator_to_array() before padding, matching
the behavior of the spread, merge, and slice operations which already
accept Traversables.