Coalesce doesn't infer result type, breaks with "can't adapt."

I'm trying to write a query condition Coalesce(x, y, z) == foo. The resulting query bombs out of psycopg with the error "can't adapt." Here x, y, and z are DBEnum values.

Edited notes from IRC:

<jamesh> The DBItem instance would be the culprit.
It is not getting translated.

<jtv> Yup. Wonder why it fails only when I use Coalesce.

<jamesh> Well, the Column.__eq__() method casts the other argument to the same Variable type that the column uses
and the Variable type for DBEnumerations knows to convert DBItems to integers before passing them down to the database.
Storm doesn't know what the Variable type for Coalesce()'s return value
so isn't doing the conversion itself.

<jtv> I thought it'd be able to infer that from its inputs... Is there an explicit cast I can apply to make this go away?

<jamesh> You could do Coalesce(x, y) == Enum.foo.value

[That worked].

<jamesh> you are right that it should be possible
but at the moment there isn't any special compilation code to do so -- it is treating Coalesce like a generic function call
and the generic function call compilation process can't determine the return type without knowing something about the function
(we could definitely improve this though)

<jamesh> it should be pretty easy to add return type awareness to things like Coalesce
and a variable_factory @property that returns args[0].variable_factory