[GHC] #8767: Add rules involving `coerce` to the libraries

[GHC]

#8767: Add rules involving `coerce` to the libraries
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        Reporter:  nomeata                       |            Owner:
            Type:  task                          |           Status:  new
        Priority:  normal                        |        Milestone:  7.8.1
       Component:  libraries/base                |          Version:
      Resolution:                                |  7.8.1-rc2
Operating System:  Unknown/Multiple              |         Keywords:
 Type of failure:  None/Unknown                  |     Architecture:
       Test Case:                                |  Unknown/Multiple
  tests/simplCore/should_run/T2110.hs            |       Difficulty:
        Blocking:                                |  Unknown
                                                 |       Blocked By:  8718
                                                 |  Related Tickets:  #2110
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Comment (by nomeata):

 > I was not suggesting that there be a blanket fmap coerce --> coerce
 rule, just that we should add the specific rules for all the individual
 (lawful) Functors that we define. I agree completely that users should be
 free to write unlawful Functors if they wish to do so.

 Ok, good. Then we fully agree :-)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8767#comment:12>
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