[GHC] #8745: GeneralizedNewtypeDeriving is still not Safe

[GHC]

#8745: GeneralizedNewtypeDeriving is still not Safe
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        Reporter:  goldfire                      |            Owner:
            Type:  bug                           |           Status:  new
        Priority:  normal                        |        Milestone:
       Component:  Compiler                      |          Version:
      Resolution:                                |  7.8.1-rc1
Operating System:  Unknown/Multiple              |         Keywords:  Safe
 Type of failure:  None/Unknown                  |     Architecture:
       Test Case:                                |  Unknown/Multiple
  should_fail/TcCoercibleFailSafe                |       Difficulty:
        Blocking:                                |  Unknown
                                                 |       Blocked By:
                                                 |  Related Tickets:
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Comment (by nomeata):

 > One way forward here is to allow role annotations on classes only when
 we have IncoherentInstances. In fact, I rather like that solution -- a
 role annotation on a class quickly leads to the possibility of
 incoherence, and this solution also neatly prohibits such annotations in
 Safe Haskell. Thoughts?

 That’s what I was about to propose as well.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8745#comment:8>
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