[GHC] #12676: Panic with piResultTys1 again

[GHC]

#12676: Panic with piResultTys1 again
-------------------------------------+-------------------------------------
           Reporter:  pjmagni        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #12634
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The code

 {{{#!hs
 {-# LANGUAGE RankNTypes, TypeFamilies #-}

 module Panic where

 class C a where
   data T a
   panic :: (forall s . T a) -> ()

 instance C () where
   data T () = T
   panic T = ()
 }}}

 produces the output

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-apple-darwin):
         piResultTys1
   R:T()
   [Any]
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12676>
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