[GHC] #14777: panic when using a function defined in terms of `error`

[GHC]

#14777: panic when using a function defined in terms of `error`
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        Reporter:  zilinc            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             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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Comment (by tdammers):

 Managed to reproduce the panic on 8.2.1; GHC HEAD builds cleanly.

 Defining the following function in `Surface.hs`, and using that instead of
 any calls to `__impossible` in `Desugar.hs`, also allows me to build
 cleanly on 8.2.1:

 {{{
 __impossibleDef :: a
 __impossibleDef = error "impossible"
 }}}

 The unmodified example also builds cleanly without optimizations.

 Which suggests that some optimization pass produces a situation that hits
 a bug somewhere, and that the difference between `__impossible` and
 `__impossibleDef` is enough to trigger or not trigger it.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14777#comment:19>
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