[GHC] #11172: Turning on optimisations produces Impossible case alternative

[GHC]

#11172: Turning on optimisations produces Impossible case alternative
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        Reporter:  nh2               |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime crash     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by thomie):

 I got this far using HEAD:

 {{{
 Preprocessing library free-4.12.1...
 [ 1 of 16] Compiling Control.Monad.Free.TH ( src/Control/Monad/Free/TH.hs,
 dist/build/Control/Monad/Free/TH.o )

 src/Control/Monad/Free/TH.hs:235:5: error:
     • The constructor ‘DataConI’ should have 3 arguments, but has been
 given 4
     • In the pattern: DataConI _ _ tname _
       In a case alternative:
           DataConI _ _ tname _ -> genFree typeSig (Just [cname]) tname
       In a stmt of a 'do' block:
         case info of {
           DataConI _ _ tname _ -> genFree typeSig (Just [cname]) tname
           _ -> fail "makeFreeCon expects a data constructor" }
 Failed to install free-4.12.1
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11172#comment:8>
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