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

[GHC]

#11172: Turning on optimisations produces Impossible case alternative
-------------------------------------+-------------------------------------
        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 fell at the first fence.  I downloaded your custom `aeson`, cd'd into
 its directory.  Then
 {{{
 bash$ cabal install --with-ghc=/home/simonpj/5builds/HEAD-4/inplace/bin
 /ghc-stage2
 Resolving dependencies...
 cabal: Could not resolve dependencies:
 trying: aeson-0.10.0.0 (user goal)
 trying: base-4.9.0.0/installed-4.9... (dependency of aeson-0.10.0.0)
 next goal: vector (dependency of aeson-0.10.0.0)
 rejecting: vector-0.11.0.0 (conflict: base==4.9.0.0/installed-4.9...,
 vector
 => base>=4.3 && <4.9)
 trying: vector-0.10.12.3
 next goal: primitive (dependency of vector-0.10.12.3)
 rejecting: primitive-0.6.1.0, 0.5.4.0 (conflict:
 base==4.9.0.0/installed-4.9..., primitive => base>=4.3 && <4.9)
 rejecting: primitive-0.5.3.0, 0.5.2.1 (conflict:
 base==4.9.0.0/installed-4.9..., primitive => base>=4.3 && <4.8)
 rejecting: primitive-0.5.1.0 (conflict: base==4.9.0.0/installed-4.9...,
 primitive => base>=4 && <4.8)
 rejecting: primitive-0.5.0.1 (conflict: base==4.9.0.0/installed-4.9...,
 primitive => base>=4 && <4.7)
 rejecting: primitive-0.5, 0.4.1, 0.4.0.1, 0.4, 0.3.1, 0.3, 0.2.1, 0.2, 0.1
 (conflict: vector => primitive>=0.5.0.1 && <0.7)
 rejecting: primitive-0.6 (conflict: base==4.9.0.0/installed-4.9...,
 primitive
 => base>=4.3 && <4.9)
 Dependency tree exhaustively searched.
 simonpj at cam-05-unx:~/tmp/aeson-ghc-8-head-workaround$
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11172#comment:5>
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