gistfile1.txt

class X x where
  a :: x -> Int

data AnX = AnX Int
instance X AnX where
  a (AnX v) = v

class Y y where
  b :: y -> Int

data AnY = AnY Int
instance Y AnY where
  b (AnY v) = v

instance Y y => X y where
  a anY = b anY

foo :: X x => x -> Int
foo x = a x

main :: IO ()
main =
  let
    anX = AnX 5
    anY = AnY 6
  in do                                                                                                                                                                                      
    print (foo anX)
    print (foo anY)

gistfile2.txt

/home/exarkun/Work/haskell/tahoe-lafs-immutable-uploader/aesctr/Main.hs:68:10: error:
    • The constraint ‘Y y’ is no smaller than the instance head ‘X y’
      (Use UndecidableInstances to permit this)
    • In the instance declaration for ‘X y’
   |
68 | instance Y y => X y where
   |          ^^^^^^^^^^