a.hs

-- ok
main :: IO ()
main = do
  let x = Nothing
  case x of
    Just 0 -> return 0
    Nothing -> return 1
  case x of
    Just "a" -> return 2
    Nothing -> return 3
  return ()

b.hs

-- b.hs:5:10:
--    No instance for (Num [Char]) arising from the literal ‘0’
--    In the pattern: 0
--    In the pattern: Just 0
--    In a case alternative: Just 0 -> return 0
main :: IO ()
main = do
  x <- return Nothing
  case x of
    Just 0 -> return 0
    Nothing -> return 1
  case x of
    Just "a" -> return 2
    Nothing -> return 3
  return ()

c.hs

{-# LANGUAGE Rank2Types #-}
-- ok
f :: (forall a. Maybe a) -> IO ()
f x = do
  case x of
    Just 0 -> return 0
    Nothing -> return 1
  case x of
    Just "a" -> return 2
    Nothing -> return 3
  return ()
main :: IO ()
main = f Nothing