[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:  highest           |            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:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 Note: Suppressing only in the absence of superclasses means that any class
 constraint that just gives you a law will **always** spring this warning.

 {{{#!hs
 class Bifunctor p => Braided p where
   braid :: p a b -> p b a

 -- | @braid . braid = id@
 class Braided p => Symmetric p

 swap :: Symmetric p => p a b -> p b a
 swap = braid
 }}}

 (This code is slightly simplified from
 http://hackage.haskell.org/package/categories-1.0.7/docs/Control-Category-
 Braided.html)

 One "solution" here is to move `swap` into the `Symmetric` class, but then
 you need to worry about it being overridden to be different than `braid`,
 which operationally can't happen in the current approach.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11369#comment:3>
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