[GHC] #8894: Clean up `Coercible` plumbing

[GHC]

#8894: Clean up `Coercible` plumbing
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        Reporter:  nomeata           |            Owner:  nomeata
            Type:  task              |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.9
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #8904
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Comment (by simonpj):

 No, wired-in things are never written to any interface file.  GHC already
 knows all about them, so there's not point in serialising them.

 Yes, the data constructor would match.  (Of course that requires a
 `Note`.)  By declaring a data type and using it as a class we are already
 being naughty in a different way, so it's really no worse.

 I have not thought about "no magic in base", although I have not
 objection. If that's the route you go, be sure to document it as the
 reason (in `GHC.Coerce`) so that we know in five years time.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8894#comment:9>
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