instance Data a => Data (Bag a) where
  gfoldl k z b = z listToBag `k` bagToList b -- traverse abstract type abstractly
  toConstr _   = abstractConstr $ "Bag("++show (typeOf (undefined::a))++")"
  gunfold _ _  = error "gunfold"
  dataTypeOf _ = mkNoRepType "Bag"

Btw, where is this instance defined?

GHCROOT/compiler/utils/Bag.lhs, lines 266-270 (current git-repo HEAD and many versions previously).

Ph.