[GHC] #8565: New GeneralisedNewtypeDeriving needs help with higher rank types

[GHC]

#8565: New GeneralisedNewtypeDeriving needs help with higher rank types
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        Reporter:  simonpj                       |            Owner:
            Type:  bug                           |           Status:  new
        Priority:  normal                        |        Milestone:
       Component:  Compiler                      |          Version:  7.6.3
      Resolution:                                |         Keywords:
Operating System:  Unknown/Multiple              |     Architecture:
 Type of failure:  None/Unknown                  |  Unknown/Multiple
       Test Case:                                |       Difficulty:
  typecheck/should_compile/T8565                 |  Unknown
        Blocking:                                |       Blocked By:
                                                 |  Related Tickets:
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Comment (by goldfire):

 I just tested this with `ImpredicativeTypes` on, and the error message was
 from the `Coercible` stuff. With another case in that code (and
 `ImpredicativeTypes`), I think this would work. The underlying roles
 machinery is certainly strong enough to handle this case.

 I'm OK if we require a user to specify `ImpredicativeTypes` for this to
 work -- it really is impredicative!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8565#comment:5>
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