[GHC] #12041: GHC panics on "print_equality ~"

[GHC]

#12041: GHC panics on "print_equality ~"
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           Reporter:  Iceland_jack   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# Language PolyKinds, TypeFamilies #-}

 import Data.Kind

 class Category (p :: i -> i -> Type) where
   type Ob p :: i -> Constraint

 data I a b
 instance Category I where
   type Ob I = (~) Int
 }}}

 fails with

 {{{
 $ ghci -ignore-dot-ghci /tmp/Error.hs
 GHCi, version 8.1.20160503: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /tmp/Error.hs, interpreted )

 /tmp/Error.hs:10:15: error:ghc: panic! (the 'impossible' happened)
   (GHC version 8.1.20160503 for x86_64-unknown-linux):
         print_equality ~

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 >
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12041>
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