[GHC] #13328: Foldable, Functor, and Traversable deriving handle phantom types badly

[GHC]

#13328: Foldable, Functor, and Traversable deriving handle phantom types badly
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:  deriving-perf
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 The role annotation should (I believe) only affect instances derived
 outside the module. So I believe the concern would be for a type that is
 defined ''using'' a type defined in another module that has a role
 annotation. But again, that matches the "what the user would likely write
 by hand" scenario: we can only do as well as the user could. Yes, it's
 true that this introduces some potential for surprises; the user won't get
 a nice error message when they make the change. Do you think this is a
 judgement call that should be made through the proposal process?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13328#comment:8>
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