QuantifiedConstraints: trouble with type family

{-# Language KindSignatures, TypeFamilies, QuantifiedConstraints, FlexibleInstances #-}

import Data.Kind

data TreeF a b = T0 | T1 a | T2 b b

-- from Data.Reify
class MuRef (a :: Type) where
  type DeRef a :: Type -> Type

class    (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree
instance (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree

fails with

$ ~/code/qc-ghc/inplace/bin/ghc-stage2 --interactive -ignore-dot-ghci 351.hs
GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( 351.hs, interpreted )

351.hs:12:10: error:
    • Couldn't match type ‘DeRef (tree xx)’ with ‘TreeF xx’
        arising from the superclasses of an instance declaration
    • In the instance declaration for ‘MuRef1 tree’
   |
12 | instance (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, no modules loaded.
Prelude> 

---

What I want to write:

{-# Language KindSignatures, TypeFamilies, QuantifiedConstraints, FlexibleInstances, DataKinds, TypeInType, MultiParamTypeClasses, UndecidableInstances, AllowAmbiguousTypes #-}

import Data.Kind

-- from Data.Reify
class MuRef (a :: Type) where
  type DeRef a :: Type -> Type

type T   = Type
type TT  = T -> T
type TTT = T -> TT

class    (forall xx. (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx)) => MuRef1 (f :: TT) (deRef1 :: TT -> TTT)
instance (forall xx. (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx)) => MuRef1 (f :: TT) (deRef1 :: TT -> TTT)

where I am trying to capture MuRef1 & DeRef1

class MuRef1 (f :: TT) where
  type DeRef1 f :: TTT
  muRef1 :: proxy (f a) -> Dict (MuRef (f a), DeRef (f a) ~ DeRef1 f a)

---

Workarounds: I think splitting the class alias & quantification does the job (I haven't tested it but it compiles), I want to know if the first two programs are meant to compile or not

{-# Language KindSignatures, TypeFamilies, QuantifiedConstraints, FlexibleInstances, DataKinds, TypeInType, MultiParamTypeClasses, UndecidableInstances, AllowAmbiguousTypes, FlexibleContexts #-}

-- ..

class    (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx) => MuRef1_ f deRef1 xx
instance (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx) => MuRef1_ f deRef1 xx

class    (forall xx. cls xx) => Forall cls
instance (forall xx. cls xx) => Forall cls

class    Forall (MuRef1_ f deRef1) => MuRef1 f deRef1
instance Forall (MuRef1_ f deRef1) => MuRef1 f deRef1

or as a regular type/constraint synonym (at the loss of partial application)

type MuRef1 f deRef1 = (forall xx. (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx))

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture