[GHC] #14799: QuantifiedConstraints: Problems with Typeable

[GHC]

#14799: QuantifiedConstraints: Problems with Typeable
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints, wipT2893
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):

 It works by creating a class alias for `Typeable`

 {{{#!hs
 class Typeable a => Typeable' a where
   typeRep' :: proxy a -> TypeRep
   typeRep' proxy = typeRep proxy

 instance Typeable a => Typeable' a

 b :: (forall xx. cls xx => Typeable' xx) => Ex cls -> TypeRep
 b (MkEx (_::xx)) = typeRep' @_ @xx Proxy
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14799#comment:1>
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