[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:  merge
        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 nomeata):

 > Are you certain of that? Since it is in wiredInTyCons it will never get
 into GHC/Types.hi, nor will it be read from the interface file, so GHCi
 could never know that in the source file it was declared with data.

 I am relatively certain, but didn’t try it out right now. I just opened a
 bug about it: #8894, maybe I’ll work on it when I have some quiet minutes,
 but not right now.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8888#comment:15>
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