This came from a review by Noah Misch a great while ago:
test=> SELECT b FROM foo ORDER BY 1 COLLATE "C";
ERROR: 42804: collations are not supported by type integer
According to SQL92, this should be supported. Do we want to bother? It
doesn't look hard to fix, so it's really only a question of whether this
would be useful, or its absence would be too confusing.