Re: 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?