[GHC] #11369: Suppress redundant-constraint warnings in case of empty classes

[GHC]

#11369: Suppress redundant-constraint warnings in case of empty classes
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        Reporter:  hvr               |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11370            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > What would this look like? Any class in the cycle being marked with the
 pragma would allow undecidable superclasses?

 This issue is when we report a class cycle.  If any class in the cycle has
 a "UndecidableSuperClasses" pragma, then yes, I suggest we do not warn.

 It'd be great to put this per-class stuff in 8.0, but someone would have
 to step up and implement it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11369#comment:12>
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