[GHC] #11449: Treat '_' consistently in type declarations

[GHC]

#11449: Treat '_' consistently in type declarations
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I would love to see (A) accepted (perhaps with some language extension).

 I think (C) should be rejected. After all, the type variable ''is'' used
 there. It's used in the type family declaration, where `a` is not
 ''bound'' -- it is ''used''.

 Accordingly,

 {{{
 class C a where
   type F a
   type F a = Int
 }}}

 should not emit any warnings. The only bound variable, `a`, is indeed
 used.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11449#comment:2>
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