Incoherent: use it with care

incoherent.hs

class Foo a where
  foo :: a -> Int

instance Foo Char where
  foo _ = 2

instance {-# INCOHERENT #-} Foo a where
    foo _ = 3

incoherent :: Char -> Bool
incoherent x = foo x /= foo' x
  where
   foo' :: (Foo a) => a -> Int
   foo' = foo