[GHC] #2850: GeneralizedNewtypeDeriving + TypeFamilies doesn't work

[GHC]

#2850: GeneralizedNewtypeDeriving + TypeFamilies doesn't work
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    Reporter:  ajd               |        Owner:                  
        Type:  feature request   |       Status:  new             
    Priority:  normal            |    Milestone:  6.12 branch     
   Component:  Compiler          |      Version:  6.10.1          
    Severity:  normal            |   Resolution:                  
    Keywords:                    |   Difficulty:  Unknown         
    Testcase:                    |           Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |  
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Comment (by pumpkin):

 I find this error puzzling because this used to work in 6.8.3 and even (I
 believe) 6.10.1. But now I'm on 6.10.3 and am trying to compile some code
 that I hadn't touched since 6.10.1, and I get this error.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2850#comment:4>
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