[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 simonpj):

 I'd like to reproduce, but I get this (with the current HEAD).  Any
 suggestions, anyone?
 {{{
 bash$ cabal install --with-ghc=/home/simonpj/5builds/HEAD-4/inplace/bin
 /ghc-stage2
 Resolving dependencies...
 cabal: Could not resolve dependencies:
 trying: impossible-case-alternative-repro-0.0 (user goal)
 trying: base-4.9.0.0/installed-4.9... (dependency of
 impossible-case-alternative-repro-0.0)
 trying: either-4.4.1 (dependency of impossible-case-alternative-repro-0.0)
 trying: semigroupoids-5.0.0.4 (dependency of either-4.4.1)
 trying: semigroupoids-5.0.0.4:+tagged
 trying: semigroupoids-5.0.0.4:+comonad
 trying: comonad-4.2.7.2 (dependency of semigroupoids-5.0.0.4:+comonad)
 trying: comonad-4.2.7.2:+distributive
 trying: comonad-4.2.7.2:+contravariant
 trying: contravariant-1.3.3 (dependency of comonad-4.2.7.2:+contravariant)
 trying: void-0.7.1 (dependency of contravariant-1.3.3)
 next goal: hashable (dependency of void-0.7.1)
 rejecting: hashable-1.2.3.3, 1.2.3.2 (conflict:
 base==4.9.0.0/installed-4.9..., hashable => base>=4.0 && <4.9)
 rejecting: hashable-1.2.3.1, 1.2.3.0, 1.2.2.0, 1.2.1.0, 1.2.0.10, 1.2.0.9,
 1.2.0.8, 1.2.0.7, 1.2.0.6, 1.2.0.5, 1.2.0.4, 1.2.0.3, 1.2.0.2, 1.2.0.1,
 1.2.0.0, 1.1.2.5, 1.1.2.4, 1.1.2.3 (conflict:
 base==4.9.0.0/installed-4.9...,
 hashable => base>=4.0 && <4.8)
 rejecting: hashable-1.1.2.2, 1.1.2.1, 1.1.2.0, 1.1.1.0, 1.1.0.0 (conflict:
 base => ghc-prim==0.5.0.0/installed-0.5..., hashable => ghc-prim<0.3)
 rejecting: hashable-1.0.1.1, 1.0.1.0, 1.0.0 (conflict: void =>
 hashable>=1.1)
 Dependency tree exhaustively searched.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11172#comment:3>
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