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

[GHC]

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

 The Coercible class is special in other ways too, there is also
 [https://phabricator.haskell.org/diffusion/GHC/browse/master/libraries
 /ghc-prim/GHC/Types.hs;54c95ffcb2cf392df33d2ec397e2301c005ca406$84 no
 syntax for ~#R]

 {{{#!hs
 class a ~R# b => Coercible (a :: k) (b :: k)
 }}}

 making it difficult to get more information about `~R#`. (#12023)

 Some of this might be worth noting in `:info`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12390#comment:1>
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