[GHC] #8503: New GeneralizedNewtypeDeriving check still isn't permissive enough

[GHC]

#8503: New GeneralizedNewtypeDeriving check still isn't permissive enough
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        Reporter:  goldfire          |            Owner:  goldfire
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:  8548
        Blocking:                    |  Related Tickets:  #8541
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Comment (by goldfire):

 I think the issue with #2850 is fixable. See that ticket.

 I actually think these knock-on effects of the GND-via-coerce decision are
 good. They're forcing us to get `Coercible` right!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8503#comment:33>
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