{-# LANGUAGE EmptyCase #-}
{-# LANGUAGE GADTs #-}
-- {-# OPTIONS_GHC -Wall #-}

module Main where

import Unsafe.Coerce

data D a where
    J :: Integer -> D Int

instance Eq (D a) where
    J i == J j = i == j

instance Num a => Num (D a) where
    fromInteger = unsafeCoerce . J . fromInteger
    {-# NOINLINE fromInteger #-}

-- >>> m
-- Just 5
m :: Maybe Integer
m = case (5 :: D Int) of
    J j -> Just j

-- >>> n
-- Nothing
n :: Maybe Integer
n = case 5 of
    5 -> Just 42
    J{} -> Just 3

main = do
  print m
  print n