{-# language GADTs #-}

data C = C

data A1 = A1
data A2 = A2
fa :: A1 -> Maybe A2 -> C
fa = undefined

data B1 = B1
data B2 = B2
fb :: B1 -> Maybe B2 -> C
fb = undefined

data Sum1 = InA1 A1 | InB1 B1
data Sum2 = InA2 A2 | InB2 B2

data Tag a where
    TagA :: Tag A2
    TagB :: Tag B2

test :: Sum1 -> Maybe Sum2 -> Either String C
test sum1 sum2 = case sum1 of
    InA1 a -> g TagA $ fa a
    InB1 b -> g TagB $ fb b
    where
        g :: Tag a -> (Maybe a -> C) -> Either String C
        g tag f = case sum2 of
            Nothing -> Right $ f Nothing
            Just x -> case (tag, x) of
                (TagA, InA2 a) -> Right $ f (Just a)
                (TagB, InB2 b) -> Right $ f (Just b)
                _ -> Left "..."

main = undefined