[GHC] #11066: Inacessible branch should be warning - otherwise breaks type soundness?

[GHC]

#11066: Inacessible branch should be warning - otherwise breaks type soundness?
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by ekmett):

 If what you're saying is that the fact that its `type family Gcd :: Nat ->
 Nat -> Nat`, rather than that other form, that is causing the problem,
 then I think I see where you're coming from.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11066#comment:31>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler