[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 RyanGlScott):

 There's one more scenario that this proposed change would wreak havoc
 with. It's quite possible for users to define unlawful `Functor` instances
 and depend on them in other derived `Functor` instances:

 {{{#!hs
 newtype Unlawful a = Unlawful Int
 instance Functor Unlawful where
   fmap f (Unlawful i) = Unlawful (i + 1)

 newtype WrapUnlawful a = WrapUnlawful (Unlawful a)
   deriving Functor
 }}}

 But under this proposal, the generated `Functor` instance for
 `WrapUnlawful` would be:

 {{{#!hs
 instance Functor WrapUnlawful where
   fmap _ = coerce
 }}}

 This would be another departure in behavior, as now `fmap`ping over a
 `WrapUnlawful` wouldn't apply `(+1)` to the field of the `Unlawful` (icky
 as it may be).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13328#comment:5>
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