[GHC] #8827: Inferring Safe mode with GeneralizedNewtypeDeriving is wrong

[GHC]

#8827: Inferring Safe mode with GeneralizedNewtypeDeriving is wrong
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        Reporter:  goldfire          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Compiler          |          Version:  7.8.1-rc2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  8226, 8745
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Comment (by ekmett):

 One option here is to just retain the status quo that
 `GeneralizedNewtypeDeriving` isn't available in `Safe` Haskell.

 Then you'd retain the invariant that GND absolutely can't be used to
 violate library invariants, and since `coerce` isn't available in `Safe`
 mode without explicit trust, the interaction of these two features goes
 away completely.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8827#comment:22>
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