data A = B | C | D
  deriving stock (Eq, Show)
  
data S (a :: A) where
  Sb :: S 'B
  Sc :: S 'C
  Sd :: S 'D

data SomeS where
  SomeS :: S a -> SomeS

mbB :: a -> Maybe (S 'B)
mbB _ = Just Sb

mbC :: a -> Maybe (S 'C)
mbC _ = Just Sc

xs :: [Maybe SomeS]
xs = [SomeS <$> mbC 1, SomeS <$> mbB 1]

ys :: [A]
ys = catMaybes $ xs <&> \case
  Just (SomeS x) -> case x of
    Sb -> Just B
    Sc -> Just C
    Sd -> Just D
  Nothing -> Nothing