[GHC] #14860: QuantifiedConstraints: Can't quantify constraint involving type family

[GHC]

#14860: QuantifiedConstraints: Can't quantify constraint involving type family
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:  wontfix           |  QuantifiedConstraints wipT2893
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Replying to [comment:11 RyanGlScott]: I don't think you need quantified
 constraints (the ticket:14822#comment:3 trick does works to solve my
 previous comment), how about this:

 {{{#!hs
 instance Typeable f => Show (ExTyFam f) where
   show (MkExTyFam x)
     | Just HRefl <- typeRep @f `eqTypeRep` typeRep @(Proxy :: Type ->
 Type)
     = show x
     | otherwise
     = ..
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14860#comment:15>
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