[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:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8128, #8740      |  Differential Rev(s):  Phab:D1454
       Wiki Page:                    |
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Comment (by tdammers):

 It turns out that `T3651` doesn't fail the right way if we turn this error
 into a warning and then compile with `-Werror`, because the core linter
 errors out first, so instead of the errorized warning, we see the core
 linter failure. Manually compiling with just `-Werror` (but not `-dcore-
 lint`) does produce the desired behavior.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11066#comment:38>
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