[GHC] #14777: panic

[GHC]

#14777: panic
-------------------------------------+-------------------------------------
           Reporter:  zilinc         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Linux
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The error message I got from GHC was:

 {{{
 <no location info>: error:
     ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.2 for x86_64-unknown-linux):
         idInfo
   t_XG0J
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/basicTypes/Var.hs:526:34 in ghc:Var

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

 How was it triggerred (in short):
 There's function definition, `f x = undefined`. If I change it to `f x =
 __impossible "Oops!"`, ghc crashes. The definition of `__impossible`
  is simply `__impossible s = error $ s ++ "some text"`.

 The code can be found here:
 https://github.com/NICTA/cogent/blob/92c40530cbcbac77469865e096dab3228a2fb92f/cogent/src/Cogent/Desugar.hs#L482
 and
 https://github.com/NICTA/cogent/blob/92c40530cbcbac77469865e096dab3228a2fb92f/cogent/src/Cogent/Compiler.hs#L30

 It seems that this bug is not limited to 8.2.2; it also happened in 8.2.1.

 I'm happy to provide more information, if someone can advice me what is
 needed. I'll also try to shrink it to a small test case.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14777>
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