[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 nomeata):

 Maybe it would work better if a symbol could have an text field attatched
 for “arbitrary text to be displayed on `:info`”, and `:info` would show
 that. It could simply say

 > Coercible is a special constraint with custom solving rules. Please see
 section ... of the user’s guide for details.

 Note that we stopped calling `Coercible` a class; as it is not a class.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12390#comment:2>
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