Illegal polymorphic or qualified type using RankNTypes and TypeFamilies

I've been slowly working on porting the llvm package to use data kinds, type families and type-nats and ran into a minor issue when trying to remove the two newtypes used for classifying values (ConstValue and Value) by introducing a new Value type parameterized by its constness.

CallArgs only accepts Value 'Variable a arguments and provides a function for casting a Value 'Const a to a Value 'Variable a. I'd like to generalize CallArgs to allow each argument to be either 'Const or 'Variable. Is this possible to encode this somehow using type families? I think it's probably doable with fundeps.

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}

data Const = Const | Variable

data Value (c :: Const) (a :: *)

type family CallArgs a :: * 
type instance CallArgs (a -> b) = forall (c :: Const) . Value c a -> CallArgs b
type instance CallArgs (IO a)   = IO (Value 'Variable a)

... which fails to compile:

/tmp/blah.hs:10:1:
    Illegal polymorphic or qualified type:
      forall (c :: Const). Value c a
    In the type instance declaration for `CallArgs'

Where the following solution works (equivalent to the legacy code), but requires the user to cast the each constant Value:

type family CallArgs' a :: * 
type instance CallArgs' (a -> b) = Value 'Variable a -> CallArgs' b
type instance CallArgs' (IO a)   = IO (Value 'Variable a)