[GENERAL] aggregates, distinct, order by, and case - why won't this work

[David G. Johnston]

This...on 9.3

SELECT array_agg(
distinct case when v % 2 = 0 then 'odd' else 'even' end
order by case when v % 2 = 0 then 1 else 2 end
)
FROM (VALUES (1), (2), (3)) val (v)

I'm not particularly irked at this though I was hoping to fix a somewhat
complex query of mine by simply adding a "DISTINCT" to the array_agg that I
am building from derived (using CASE) data.

I am curious to the reason for the limitation, particularly as it would
relate to this specific instance.

Any givers?

Thank!

David J.