[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:  8.2.1
       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 simonpj):

 * Concerning the original ticket: I'd be perfectly happy if it became a
 warning.  I agree that it's a bit odd if a program is accepted, but then
 after one beta-step it is rejected.  (I'm not sure I'd call it type-
 unsound, which to me means that it's not rejected at all, but crashes at
 runtime.)

 * Concerning comment:15, good point. I think it might be quite tricky to
 make the 'deriving' mechanism predict which branches would be
 inaccessible; but perhaps possible.   A significant advantage of making
 the error into a warning is that we could then ignore the warning in
 deriving code (we already ignore others).

 Happy to advise if someone would like to take this up.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11066#comment:16>
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