[GHC] #8888: Document Coercible in user's guide

[GHC]

#8888: Document Coercible in user's guide
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        Reporter:  goldfire          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Documentation     |          Version:  7.8.1-rc2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by simonpj):

 "`coercibleTyCon` is added to `ghcPrimExports` in `PrelInfo`".  Wow!  That
 is truly unique!   `Coercible` is defined and exported (in the interface
 file) by `GHC.Types`, '''and''' is exported by `GHC.Prim`.  I can imagine
 all sorts of confusion can arise from that.  I'm astonished it works.
 And, despite its unique weirdness, there is no comment with
 `ghcPrimExports`.

 None of this happens for `(~)`.  Why it needed for `Coercible`?

 After all, it's in the `wiredInTyCons` in `TysWiredIn`. Why does it also
 need to be in `ghcPrimExports` at all?  Why not just remove it?  (And
 import it from `GHC.Types` in `Data.Coerce`).  There may be some Deep
 Reason for all this strangeness, but I can't see it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8888#comment:7>
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