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

[GHC]

#8894: Clean up `Coercible` plumbing
-------------------------------------+-------------------------------------
              Reporter:  nomeata     |            Owner:  nomeata
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.9
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:  #8904
             Test Case:              |
              Blocking:              |
Differential Revisions:  Phab:D300   |
-------------------------------------+-------------------------------------
Changes (by nomeata):

 * differential:   => Phab:D300


Comment:

 I gave it a shot (Phab:D300), but I get
 {{{
   HC [stage 1] utils/dll-split/dist-install/build/tmp/dll-split
 /home/jojo/build/haskell/ghc-validate/libraries/base/dist-install/build
 /libHSbase_ESD4aQEEWwsHtYJVc1BwtJ-ghc7.9.20141002.so: undefined reference
 to `base_DataziCoerce_DZCCoercible_con_info'
 }}}

 Is there really a data constructor being generated for an empty class?
 (Feel free to continue discussing this on Phab:D300).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8894#comment:13>
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