[GHC] #11582: Redundant constraints warning complains that (a ~ b) is redundant

[GHC]

#11582: Redundant constraints warning complains that (a ~ b) is redundant
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc2
  checker)                           |
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         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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Comment (by simonpj):

 I see.  Yes that is true.  Sort of.  Removing a type class with functional
 dependencies might have the same effect.  Or a type class with a
 superclass that had functional depencies.  Or a type class with a
 superclass equality like
 {{{
 class (a~b) => C a b where ...
 }}}
 And with higher-kinded stuff I don't think we can predict with precision.

 I suppose you could have `-fwarn-redundant-constraints-for-definition` and
 `-fwarn-redundant-constraints-for-call-sites`, to say what level of
 redundancy you wanted to report or at least make best-efforts to report
 (see above).

 But I do see what you mean, and it might be good to have such an example
 in the user manual.

 Also if you felt like implementing per-signature control that would be
 cool.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11582#comment:3>
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