[GHC] #12142: -Wredundant-constraints warns about constraints introduced via type synonyms.

[GHC]

#12142: -Wredundant-constraints warns about constraints introduced via type
synonyms.
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           Reporter:  scshunt        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Linux
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I ran into this when using lens, here's a simplied example:

 {{{#!hs
 l :: Getter [a] Int
 l = to length
 }}}

 In this case, Getter introduces constraints (Functor f, Contravariant f),
 but the Functor constraint is not used. This leads to a warning from
 -Wredundant-constraints:

 {{{
 <interactive>:1:6: warning: [-Wredundant-constraints]
     • Redundant constraint: Functor f
     • In the type signature for:
            l :: (Contravariant f, Functor f) => (Int -> f Int) -> [a] -> f
 [a]
 }}}

 This shouldn't warn, because the constraints were introduced by type
 synonym and so may well be opaque to the user.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12142>
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