[GHC] #11066: Inacessible branch should be warning - otherwise breaks type soundness?

[GHC]

#11066: Inacessible branch should be warning - otherwise breaks type soundness?
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        Reporter:  rrnewton          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8128, #8740      |  Differential Rev(s):  Phab:D1454
       Wiki Page:                    |
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Changes (by adamgundry):

 * cc: adamgundry (added)
 * related:  #8128 => #8128, #8740


Comment:

 I'd like to see this: it would be particularly useful for code generated
 by standalone deriving (see related tickets).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11066#comment:10>
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