[GHC] #10982: Warn about unused pattern variables in type families

[GHC]

#10982: Warn about unused pattern variables in type families
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        Reporter:  jstolarek         |                Owner:  msosn
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11098            |  Differential Rev(s):  Phab:D1576
       Wiki Page:                    |
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Changes (by jstolarek):

 * related:   => #11098


Comment:

 This looks like a manifestation of #11098. I tried fixing it by "making
 renamer do its job" but got stuck because I didn't know how to tell
 whether a type variable was introduced using a forall or not - see
 [ticket:11098#comment:3]. But this was before wildcard refactor patch
 (Phab:D1428). Now, looking at Note [HsType binders] in HsTypes, I think it
 is easy to tell whether a type variable was quantified explicitly or not.
 Michał, does this allow you to make progress?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10982#comment:7>
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