[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:  8.4.1
       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 dfeuer):

 I wonder if ekmett's problem lies in the somewhat bogus type family
 definition:

 {{{#!hs
 type family Gcd :: Nat -> Nat -> Nat
 }}}

 An instance of the family would have to be a ''constructor'' of kind `Nat
 -> Nat -> Nat`. There is no such thing, so GHC may reject the notion under
 some circumstances. This is just speculation, though.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11066#comment:22>
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