user-specified instance

error

src/Some.hs:17:16-77: error:
    • Class ‘Typeable’ does not support user-specified instances
    • In the quantified constraint ‘forall x. c x => Typeable x’
      In the type signature:
        extractSome :: (Typeable a, forall x. c x => Typeable x) =>
                       Some c -> Maybe a
   |
26 | extractSome :: (Typeable a, forall x. c x => Typeable x) => Some c -> Maybe a
   |  

Some.hs

{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE ConstraintKinds #-}
module Some where

import Data.Typeable
import Data.Kind


data Some c where
    Some :: c a => a -> Some c

extractSome :: (Typeable a, forall x. c x => Typeable x) => Some c -> Maybe a
extractSome (Some a) = cast a