Feature request: Warn about aggregating over possibly incompatible types

[nbt...@nbtrap.com]

We recently pushed to our production environment a feature that involved
summing (with django.db.models.Sum) over CharField entries containing
numeric text. It ended up not working correctly since the postgresql
SUM function can't handle textual arguments, whereas sqlite's apparently
can.

A nice feature that might have saved us the trouble would be to emit
warnings when running an aggregation function with types that don't work
with all of the db backends.

Any objections to me filing a feature request for this?