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

[GHC]

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

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

 produces the warning

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

 Since GHC can detect there is a redundancy, it would be nice it also told
 me which constraint was redundant. In #9939, I suggested that the warning
 include something to the effect of `(Num a) is implied by (Integral a)`.
 Lets make this as easy for the user as possible.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12544>
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