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

[GHC]

#8767: Add rules involving `coerce` to the libraries
-------------------------------------------------+-------------------------
        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 still believe that people should be free to write non-law-abiding
 instances (or maybe law-abiding up to some interface), and the compiler
 should not interfere with that.

 We should simply suggest them to add a `fmap coerce = coerce` rule for
 their particular fmap if they care.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8767#comment:9>
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