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

[GHC]

#11369: Suppress redundant-constraint warnings in case of empty classes
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by bgamari):

 > If someone is going to add class-specific pragmas, making
 `UndecidableSuperClasses` work on a per-class (rather than per-module)
 basis would be good.

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

 If we are going to do this we may want to consider doing it before the 8.0
 release. Moving from per-module to per-class modules after the per-class
 pragmas are in the wild may be rather painful (judging by the move from
 `OverlappingInstances` to `{-# OVERLAPPING #-}`, although perhaps
 `UndecidableSuperClasses` won't see as wide use).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11369#comment:11>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler