QuantifiedConstraints: Can't be RHS of type family instances

Here's a type family that I tried to write using QuantifiedConstraints.

{-# LANGUAGE TypeOperators         #-}
{-# LANGUAGE RankNTypes            #-}
{-# LANGUAGE KindSignatures        #-}
{-# LANGUAGE DataKinds             #-}
{-# LANGUAGE TypeFamilies          #-}
{-# LANGUAGE QuantifiedConstraints #-}
module QCTypeInstance where

import GHC.Exts (Constraint)

type family   Functors (fs :: [(* -> *) -> * -> *]) :: Constraint
type instance Functors '[] = (() :: Constraint)
type instance Functors (t ': ts) = (forall f. Functor f => Functor (t f), Functors ts)

Unfortunately, GHC complains that it's illegal to have polymorphism on the right hand side of a type instance definition.

$ ../ghc-wip/T2893/inplace/bin/ghc-stage2 --interactive QCTypeInstance.hs 
GHCi, version 8.5.20180322: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling QCTypeInstance   ( QCTypeInstance.hs, interpreted )

QCTypeInstance.hs:13:15: error:
    • Illegal polymorphic type:
        forall (f :: * -> *). Functor f => Functor (t f)
    • In the type instance declaration for ‘Functors’
   |
13 | type instance Functors (t ': ts) = (forall f. Functor f => Functor (t f), Functors ts)
   |               ^^^^^^^^

Would it be possible to lift this restriction and allow quantified constraints as right hand sides of type family instances? Or are there fundamental difficulties with what I'm trying to do?

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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