[GHC] #9380: ghc generates seemingly incorrect code

[GHC]

#9380: ghc generates seemingly incorrect code
-------------------------------------+-------------------------------------
       Reporter:  qnikst             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:  Incorrect
     Blocked By:                     |  result at runtime
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 GHC generates a wrong code for the case-match on GADT with polymorphic
 type,

 here is a minimal example attached, expected results are 'A' in all test
 cases
 (this is case for ghc-HEAD), however in:
 test0 - result is missing as matches are removed from code (according to
 core)
 test1 - returns A as expected
 test2 - output 'o_O' without any checks

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9380>
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