[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:  ekmett
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Core Libraries    |                 Version:  7.9
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:  8718              |               Test Case:
 Related Tickets:  #2110             |  tests/simplCore/should_run/T2110.hs
                                     |                Blocking:
                                     |  Differential Revisions:
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Comment (by htebalaka):

 I'm a little out of my depth with rewrite rules, but is there anything
 preventing rules being associated with a typeclass? Then you could have
 zero method typeclasses like
 {{{
 class Functor f => LawfulFunctor f where
     {-# RULES "fmap/coerce" fmap coerce = coerce #-}
 }}}
 and then any datatype that wants the rule just implements the typeclass,
 which is trivial. It would also address dfeuer's comment about lawfullness
 in constraints, though it would also generate a lot of noise in
 documentation (and might inherit the same issue with typeclasses that they
 can be hard to refactor).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8767#comment:27>
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