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

[GHC]

#11172: Turning on optimisations produces SEGFAULT or Impossible case alternative
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        Reporter:  nh2               |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  highest           |            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 Simon Peyton Jones <simonpj@…>):

 In [changeset:"514bac264cb60db26ff9da10c6b79c3f5bd6e96d/ghc" 514bac2/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="514bac264cb60db26ff9da10c6b79c3f5bd6e96d"
 Fix combineIdenticalAlts

 This long-standing bug in CoreUtils.combineIdenticalAlts
 was shown up by Trac #11172. The effect was that it returned
 a correct set of alternatives, but a bogus set of "impossible
 default constructors".  That meant that we subsequently
 removed all the alternatives from a case, and hence ended
 up with a bogusly empty case that should not have been empty.

 See Note [Care with impossible-constructors when
 combining alternatives] in CoreUtils.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11172#comment:26>
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