A.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE GADTs #-}
module A where

a0 :: EqBox A
a0 = EqBox A

data EqBox a where EqBox :: Eq a => a -> EqBox a

data A = A

instance Eq a where
    (==) _ _ = False

Main.hs

import A

instance Eq A where
    (==) _ _ = True

a1 :: EqBox A
a1 = EqBox A


f :: EqBox a -> EqBox a -> Bool
f (EqBox a) (EqBox b) = a == b


main = do
    print $ f a0 a1
    print $ f a1 a0
-- main prints True\nFalse, saying that the Eq dictionary from the second argument is used