List of items of types restricted by typeclass

I'm trying to encode a list of items which have types restricted to be instances of some type class:

{-# LANGUAGE RankNTypes, TypeSynonymInstances, LiberalTypeSynonyms #-}
module Test where

class Someable a where
  some :: a -> String

data Some = Some String

type SomeGroup = forall a. Someable a => [a]

instance Someable Some where
  some (Some v) = v

instance Someable SomeGroup where
  some (x:xs) = (some x) ++ ", " ++ (some xs)

main = do
  putStrLn $ show.some [Some "A", [Some "B", Some "C"]]

But compilation fails with error:

Test.hs:14:10:
    Illegal polymorphic or qualified type: SomeGroup
    In the instance declaration for `Someable SomeGroup'

It seems I even failed to define instance for type synonymous...

I'm aware of heterogenous collections wiki article, but want to know why exactly my approach doesn't work — it seems natural to me to define type by restricting collection only to contain items with types which is instance of some type class.