[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 question about `fmap` strictness looks like a red herring to me.
 There's no real difference between the empty case definition and the
 `coerce` definition. The illegitimate `fmap` is of very little interest to
 me. Someone who cares about preserving their map count or whatever just
 won't be able to use `Functor` deriving for types explicitly mentioning
 their bogus one.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13328#comment:6>
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