{-# LANGUAGE ExistentialQuantification #-}

data Foo = forall a . Bar
  { value :: a
  }

-- value' :: Foo -> a
-- value' (Bar x) = x
{-| Couldn't match expected type ‘a’ with actual type ‘a1’
      ‘a1’ is a rigid type variable bound by
        a pattern with constructor: Bar :: forall a. a -> Foo
|-}