[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:  7.10.1
             Component:              |          Version:  7.9
  libraries/base                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:  8718
       Type of failure:              |  Related Tickets:  #2110
  None/Unknown                       |
             Test Case:              |
  tests/simplCore/should_run/T2110.hs|
              Blocking:              |
Differential Revisions:              |
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Comment (by dmcclean):

 Could the compiler always generate `(Functor f, Coercible a b) =>
 Coercible f a -> Coercible f b` instances that magically either `fmap
 coerce` or `coerce` depending on the role of f's parameter? Wouldn't the
 concrete type of `f` be known by the time it came around to generating the
 instance?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8767#comment:19>
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