[GHC] #12544: Improved warning for redundant-constraints

[GHC]

#12544: Improved warning for redundant-constraints
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        Reporter:  crockeea          |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9939             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 I think this already happens.

 {{{
 foo :: (Num a, Integral a) => a -> a
 foo = (1 +)
 }}}

 gives the warning

 {{{
 testtc.hs:1:1: warning: [-Wredundant-constraints]
     • Redundant constraint: Num a
     • In the type signature for:
            foo :: (Num a, Integral a) => a -> a
 }}}

 Both constraints are listed in your example as neither is used in the body
 of the function.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12544#comment:3>
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