[GHC] #10635: -fwarn-redundant-constraints should not be part of -Wall

[GHC]

#10635: -fwarn-redundant-constraints should not be part of -Wall
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        Reporter:  Lemming           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.11
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9939, #9973,     |  Differential Rev(s):
  #10100, #10183, #11370             |
       Wiki Page:                    |
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Comment (by konn):

 I need more flexible control on `-Wredundant-constraints`, too.
 I'm currently maintaining library to provide algebraic structures.
 In such a situation, we need '''empty''' class without any member function
 to express the additional algebraic constraint.

 Suppose we have following classes:
 {{{
 class Multiplicative r where
   (*) :: r -> r -> r

 class Commutative r -- without any member functions!
 }}}

 Until here, everything goes well.
 Suppose we want to write algorithm relying on the commutativity of
 multiplication:

 {{{
 myGreatAlgorithm :: (Commutative r) => r -> r
 myGreatAlgorithm = ...
 }}}

 Then GHC warns about `Commutative r` as a "redundant constraint", although
 it is essentially needed to work algorithm properly.
 Since `Commutative` class doesn't have any member functions, workaround
 adding "dead code", which is introduced in GHC users' guide, doesn't work
 at all.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10635#comment:23>
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