FILTER/WITHIN GROUP vs. expressions; is a HINT possible here?

Folks:

SELECT
device_id,
count(*)::INT as present,
count(*)::INT FILTER (WHERE valid) as valid_count,
mode()::INT WITHIN GROUP (order by val) as mode,
percentile_disc(0.5)::INT WITHIN GROUP (order by val)
as median
FROM dataflow_0913
GROUP BY device_id
ORDER BY device_id;

ERROR: syntax error at or near "FILTER"
LINE 4: count(*)::INT FILTER (WHERE valid)
as valid_count,

The error is right, that's invalid syntax. I can't insert a ::INT
between the aggregate() and FILTER. However, the error message is also
rather confusing to the user; they're likely to look for their mistake
in the wrong place. The same goes for WITHIN GROUP (and OVER, too, I
think).

Is there some kind of possible HINT we could add to make this easier to
debug?

--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com