[GHC] #10534: Data families + Coercible = unsafeCoerce

[GHC]

#10534: Data families + Coercible = unsafeCoerce
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        Reporter:  goldfire          |                   Owner:
            Type:  bug               |                  Status:  merge
        Priority:  highest           |               Milestone:  7.10.2
       Component:  Compiler          |                 Version:  7.10.2-rc1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |  typecheck/should_fail/T10534
                                     |                Blocking:
                                     |  Differential Revisions:  Phab:D992
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Comment (by simonpj):

 This pne-line change _really_ needs more comments.  But once the next
 patch lands it'll have them, and we can do without on the 7.10 branch

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10534#comment:4>
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