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

[GHC]

#10982: Warn about unused pattern variables in type families
-------------------------------------+-------------------------------------
           Reporter:  jstolarek      |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.11
           Keywords:  newcomer       |  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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 Now that we have wildcards in type families (#3699) I want to be warned
 about unused pattern variables in the same way I am warned about them at
 the term level. For example this should cause a warning that `b` is not
 used:

 {{{#!hs
 type family F a b
 type instance F a b = a
 }}}

 Prefixing unused variable with an underscore should suppress the warning:

 {{{#!hs
 type family F a b
 type instance F a _b = a
 }}}

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