import Unsafe.Coerce (unsafeCoerce)

class Foo r

f :: (Eq a, Foo a) => a -> a
f a = a

g :: Eq a => a -> a
g = unsafeCoerce f

f2 :: (Eq a, Num a) => a -> a
f2 a = a

g2 :: Eq a => a -> a
g2 = unsafeCoerce f2

main = print $ g "abb"