[GHC] #12785: GHC panic, TypeFamily in equality constraint

[GHC]

#12785: GHC panic, TypeFamily in equality constraint
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           Reporter:  heisenbug      |             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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 Compiling the attached module I get
 {{{
 $ ghc --interactive Tree.hs
 GHCi, version 8.1.20161030: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Tree             ( Tree.hs, interpreted )

 Tree.hs:28:63: error:ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.1.20161030 for x86_64-apple-darwin):
         tcTyVarDetails
   cobox0_a1eK :: (m_a1ej[sk:2] :: Peano)
                  ~#
                  ('S n_a1eJ[ssk:3] :: Peano)
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1076:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1080:37 in
 ghc:Outputable
         pprPanic, called at compiler/basicTypes/Var.hs:457:22 in ghc:Var

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

 When switching the call to `SBranchX` in line 29 to a call to `SBranch`
 the program compiles. So I suspect that the problem is rooted in the extra
 constraint attached to constructor `SBranchX`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12785>
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