[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):

 Replying to [comment:45 simonpj]:
 > > But that doesn't seem right.
 >
 > It seems fine to me, arguably.  The equality `a~b` is not needed;
 running `unsafe3` will never seg-fault.

 Right, of course... it's nonsensical code, but it won't blow up. I just
 found it peculiar that the "Inaccessible code" warning doesn't fire at all
 anymore.

 > BTW, if it's now a warning do we have a flag to control whether the
 warning is enabled?  We should.

 Not yet, but I was going to add one. Just didn't want to put in that
 effort while it was still unclear whether we go through with this.

 I would suggest that we turn it on by default though; it's generally a
 useful warning to have, and disabling it is probably something you'd only
 want to do when you know what you're doing.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11066#comment:46>
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