{-# language TypeFamilies, AllowAmbiguousTypes, DataKinds, UndecidableSuperClasses, UndecidableInstances, TypeFamilyDependencies, QuantifiedConstraints #-}
module M where

import Data.Kind (Type)
import Data.Proxy (Proxy (Proxy))

class C a where

instance C Int

type family F x :: Type -> Type

class    C (F x a) => CF x a
instance C (F x a) => CF x a

-- workaround
fun'' :: forall x. (forall a. C a => CF x a) => Proxy x -> ()
fun'' = fun0 @x @Int

-- rejected: "Illegal type synonym family application ‘F x’ in instance"
-- fun0 :: (C Int => C (F x Int)) => Proxy x -> ()
-- fun0 = undefined

-- compiles fine
fun0 :: C (F x a) => Proxy x -> ()
fun0 = undefined

-- another copy of the workaround
fun2'' :: (forall a. C a => CF x a) => Proxy x -> ()
fun2'' = fun20

-- compiles fine
fun20 :: C (F x Int) => Proxy x -> ()
fun20 = undefined