[GHC] #12390: List rules for `Coercible` instances

[GHC]

#12390: List rules for `Coercible` instances
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        Reporter:  Iceland_jack      |                Owner:  tellah
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  ghci059
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3634
       Wiki Page:                    |
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Comment (by duog):

 The haddocks for Data.Coerce say it is a class, and include the ~R#
 constraint. Should this string also appear in the haddocks? Attaching the
 haddocks for Data.Coerce from HEAD.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12390#comment:9>
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