[GHC] #11450: Associated types at wrong type in instance

[GHC]

#11450: Associated types at wrong type in instance
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       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 rwbarton):

 Replying to [comment:11 RyanGlScott]:
 > [...]
 >
 > Then we'd have to do something like this:
 >
 > {{{#!hs
 > instance Warning (Either a b) where
 >   type T (Either a b) = a
 > }}}
 >
 > But this results in a warning on GHC 8.0!
 >
 > {{{
 >     Defined but not used: type variable ‘b’
 > }}}
 >
 > And if you changed `b` to `_`, it would no longer follow the rules
 described above.

 GHC 8.0 does warn about that but it shouldn't. See #11451.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11450#comment:14>
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