[GHC] #11451: Inconsistent warnings for unused binders

[GHC]

#11451: Inconsistent warnings for unused binders
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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 simonpj):

 I see what you mean.  After all, you could imagine that it is "really"
 {{{
 instance forall a. C (Maybe a) where ...
 }}}
 BUT for `type instance` declarations we do allow
 {{{
 type instance F (Maybe _) = Int
 }}}
 even though it's "really"
 {{{
 type instance forall a. F (Maybe a) = Int
 }}}
 And here it really is persuasive to use `_`, in just the same way that we
 do in patterns in term-level function definitions.

 Why should the type patterns in a class instance declaration be treated
 differently than those in a type-family instance declaration?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11451#comment:4>
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