[GHC] #10538: Simpilifier produces empty case in core, segfaults at runtime.

[GHC]

#10538: Simpilifier produces empty case in core, segfaults at runtime.
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        Reporter:  sopvop            |                   Owner:  bgamari
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:  7.10.2
       Component:  Compiler          |                 Version:  7.10.2-rc1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime crash     |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"023a0ba938b69bbb89cb2ce48a07459b07783391/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="023a0ba938b69bbb89cb2ce48a07459b07783391"
 Care with impossible-cons in combineIdenticalAlts

 This was a nasty, long-standing bug exposed in Trac #10538.
 Symptoms were that we had an empty case
    case (x :: Either a) of {}
 Core Lint correctly picked this bogus code up.

 Here is what happened

 * In SimplUtils.prepareAlts, we call
         filterAlts
   then
         combineIdenticalAlts

 * We had    case x of { Left _ -> e1; Right _ -> e1 }

 * filterAlts did nothing, but correctly retuned imposs_deflt_cons
   saying that 'x' cannot be {Left, Right} in the DEFAULT branch,
   if any (there isn't one.)

 * combineIdentialAlts correctly combines the identical alts, to give
      case x of { DEFAULT -> e1 }

 * BUT combineIdenticalAlts did no adjust imposs_deft_cons

 * Result: when compiling e1 we did so in the belief that 'x'
   could not be {Left,Right}.  Disaster.

 Easily fixed.

 (It is hard to trigger; I can't construct a simple test case.)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10538#comment:5>
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