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

[GHC]

#3118: ghc: panic! (the 'impossible' happened): Missing alternative
------------------------------------------------+---------------------------
    Reporter:  calvins                          |        Owner:         
        Type:  bug                              |       Status:  closed 
    Priority:  normal                           |    Milestone:         
   Component:  Compiler                         |      Version:  6.10.1 
    Severity:  normal                           |   Resolution:  fixed  
    Keywords:                                   |   Difficulty:  Unknown
    Testcase:  simplCore/should_compiler/T3118  |           Os:  Linux  
Architecture:  x86_64 (amd64)                   |  
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Changes (by simonpj):

  * testcase:  => simplCore/should_compiler/T3118
  * status:  new => closed
  * resolution:  => fixed

Comment:

 Aha.  We managed to reproduce it.  Fixed by this
 {{{
 Thu Apr  2 16:28:34 BST 2009  simonpj at microsoft.com
   * Fix Trac #3118: missing alternative

   This patch fixes a rather obscure bug, whereby it's possible
   for (case C a b of <alts>) to have altenatives that do not inclue
   (C a b)!  See Note [Unreachable code] in CoreUtils.


     M ./compiler/basicTypes/MkId.lhs -2 +6
     M ./compiler/coreSyn/CoreUtils.lhs -11 +42
     M ./compiler/simplCore/Simplify.lhs -27 +28
     M ./compiler/specialise/SpecConstr.lhs -1 +3
 }}}

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3118#comment:5>
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