{-# LANGUAGE FlexibleContexts #-}

module FailDueToGivenOverlapping where

class C a where

class D a where
 dop :: a -> ()


instance C a => D [a]

-- should succeed since we can't learn anything more for 'a'
foo :: (C a, D [Int]) => a -> ()
foo x = dop [x]


class E a where
 eop :: a -> ()

instance E [a] where
 eop = undefined

-- should fail since we can never be sure that we learnt
-- everything about the free unification variable.
bar :: E [Int] => () -> ()
bar _ = eop [undefined]