[GHC] #8631: Need ImpredicativeTypes for GeneralizedNewtypeDeriving?

[GHC]

#8631: Need ImpredicativeTypes for GeneralizedNewtypeDeriving?
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        Reporter:  goldfire          |            Owner:
            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:
        Blocking:                    |  Related Tickets:
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Comment (by nomeata):

 It should have been [95ba5d81/ghc]: We check at deriving time that the
 `Coercible` instances are solvable, and report an appropriate error
 message; any time that there is an error message mentioning `coercible`
 (or deriv-generated code in general), that is a bug.

 So unless turning on `ImpredicativeTypes` for deriv’ed code solve the
 problem, we need to check for this already at deriving phase, and report
 an error. What would that error say?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8631#comment:5>
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