[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):

 I've been looking at this for a little while, and AFAICT, we have two
 actionable points at hand here:

 1. "Inaccessible code" is currently an error, but it would be more
 reasonable to make it a warning.
 2. Programs exist that the type checker currently rejects, even though we
 would like them to be accepted. It seems that there is no consensus on the
 exact conditions for this though.

 I believe that #1 is easy to solve, and would produce immediate benefit,
 while #2 needs better understanding of the actual problem. So I would
 suggest we tackle #1 now, and continue the discussion on #2.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11066#comment:35>
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