[GHC] #15593: QuantifiedConstraints: trouble with type family

[GHC]

#15593: QuantifiedConstraints: trouble with type family
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints,
                                     |  ConstraintKinds
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Oh dear can of worms, what about the workaround then: does that compile on
 HEAD? I will have to read that ticket

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

 import Data.Kind

 class MuRef (a :: Type) where
   type DeRef a :: Type -> Type

 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
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15593#comment:2>
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