[GHC] #11523: Infinite Loop when mixing UndecidableSuperClasses and the class/instance constraint synonym trick.

[GHC]

#11523: Infinite Loop when mixing UndecidableSuperClasses and the class/instance
constraint synonym trick.
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        Reporter:  ekmett            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |             Keywords:
      Resolution:                    |  UndecidableSuperClasses
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  polykinds/T11523
      Blocked By:                    |             Blocking:
 Related Tickets:  #11480            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 I would have thought that if we're not hitting a fixed point in the
 attached example, we'd have a similar breakage in the example in the
 description, because we'd see the same tower of Nat's there, but that
 stripped down version compiles just fine.

 Is this something that only comes up when I ''use'' constraints like this
 somehow?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11523#comment:19>
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