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

[GHC]

#8894: Clean up `Coercible` plumbing
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        Reporter:  nomeata           |            Owner:
            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:
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Comment (by Joachim Breitner <mail@…>):

 In [changeset:"014223745186fc0ca6afb9938227df9ce7d28b38/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="014223745186fc0ca6afb9938227df9ce7d28b38"
 Test case: :info Coercible in GHCi

 This prepares against future breakage, especially if #8894 is tackled.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8894#comment:1>
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