[GHC] #12966: GHC panics with forall and constraint

[GHC]

#12966: GHC panics with forall and constraint
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           Reporter:  xcmw           |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 {{{
 λ> type Maybeify c = forall d. (c d) => ((~) (Maybe d))

 <interactive>:2:39: error:ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-apple-darwin):
         print_equality ~

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

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12966>
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