[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 think that `-fwarn-redundant-constraints` should be still a part of
 `-Wall`, but we need some way to suppress warning for ''intensionally
 tightened'' constraints.

 In such a viewpoint, I like something like Simon's `{-#
 NO_WARN_REDUNDANT_CONSTRAINTS #-}` annotation.
 I think it's likely to happen that, in the same function, some constraint
 is intentionally tightened but other is not.
 So I think that it's ideal that one can specify ''necessity'' for each
 constraint.

 Example:

 {{{
 flipAdd :: (Multiplicative r, {-# NON_REDUNDANT #-} Abelian r) => r -> r
 -> r
 flipAdd x y = x + y
 }}}

 Compiler should warn `Multiplicative r` as redundant since it doesn't use
 any multiplication, but `Abelian r` should be unwarned because it's marked
 as ''non-redundant''.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10635#comment:27>
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