[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 nh2):

 Simon, for reproducing with GHC 8 HEAD it seems a little more effort is
 needed, but it seems to work for me this way:

 * From my repro, use the `ghc-8-head` branch instead
 (https://github.com/fpco/impossible-case-alternative-
 repro/tree/ghc-8-head); use `cabal install --with-ghc=...` as you did, it
 will now break at compiling `aeson` because that's not yet GHC 8 ready;
 ignore that issue

 * Use my custom `aeson` version from
 https://github.com/nh2/aeson/tree/ghc-8-head-workaround, which should
 compile with `cabal install --with-ghc=...`

 Then you should be able to `ghc --make` with GHC head.

 Please let me know if it works!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11172#comment:4>
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