[GHC] #14968: QuantifiedConstraints: Can't be RHS of type family instances

[GHC]

#14968: QuantifiedConstraints: Can't be RHS of type family instances
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        Reporter:  josef             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9269             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 This compiles

 {{{#!hs
 type family   Functors (fs :: [(Type -> Type) -> Type -> Type]) ::
 Constraint
 type instance Functors '[] = (() :: Constraint)
 type instance Functors (t:ts) = (LvlUp Functor t, Functors ts)

 class    (forall x. cls x => cls (f x)) => LvlUp cls f
 instance (forall x. cls x => cls (f x)) => LvlUp cls f
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14968#comment:9>
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