[GHC] #3927: Incomplete/overlapped pattern warnings + GADTs = inadequate

[GHC]

#3927: Incomplete/overlapped pattern warnings + GADTs = inadequate
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              Reporter:  simonpj     |            Owner:  simonpj
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  6.12.1
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:  #4139
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by nh2):

 I'm having the same problem, `-Wall` reporting `Patterns not matched: Step
 _ (End _)` for some code but if I put that match in, I get `Inaccessible
 code in a pattern with constructor`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3927#comment:39>
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