[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 goldfire):

 I think it's best to require the variables to be the same. They are the
 same under the hood, so they should be the same on top of the hood, too.
 The fact that we didn't require this previously is a bug, in my opinion.

 I don't see how the example in comment:5 argues otherwise. Yes, instances
 should allow non-linear uses of variables, but I don't see that as related
 to this overall issue.

 I could see someone arguing about spurious code breakage here, but I'm not
 too worried. And the change they would have to make is fully backward
 compatible.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11450#comment:6>
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