[GHC] #3118: ghc: panic! (the 'impossible' happened): Missing alternative

[GHC]

#3118: ghc: panic! (the 'impossible' happened): Missing alternative
-------------------------------+--------------------------------------------
    Reporter:  calvins         |        Owner:         
        Type:  bug             |       Status:  new    
    Priority:  normal          |    Milestone:         
   Component:  Compiler        |      Version:  6.10.1 
    Severity:  normal          |   Resolution:         
    Keywords:                  |   Difficulty:  Unknown
    Testcase:                  |           Os:  Linux  
Architecture:  x86_64 (amd64)  |  
-------------------------------+--------------------------------------------
Changes (by simonpj):

  * difficulty:  => Unknown

Comment:

 Very odd.  This happens in the situation where GHC's optimiser encounters
 {{{
    case (OutputFormat ...) of
       Help ->
       Debug ->
       Version ->
 }}}
 with no other alternatives, not even a default.  That can't happen in
 source code, since GHC adds an invisible default, but it can happen
 internally if the optimiser proves that no other alternatives can happen.
 But here another alternative does!  That's a bug. Needs investigation.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3118#comment:1>
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