[GHC] #11480: UndecidableSuperClasses causes the compiler to spin with UndecidableInstances

[GHC]

#11480: UndecidableSuperClasses causes the compiler to spin with
UndecidableInstances
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        Reporter:  ekmett            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |             Keywords:  PolyKinds,
      Resolution:                    |  UndecidableSuperClasses
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10318            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 Sorry, I pasted the first example after I removed the
 `UndecidableInstances`  from the code.

 With that turned on it spins.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11480#comment:5>
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