{-# 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, sum2) of
  (InA1 a1, Nothing) -> Right $ fa a1 Nothing
  (InA1 a1, Just (InA2 a2)) -> Right $ fa a1 (Just a2)
  (InB1 b1, Nothing) -> Right $ fb b1 Nothing
  (InB1 b1, Just (InB2 b2)) -> Right $ fb b1 (Just b2)
  _ -> Left "..."

main = undefined