[GHC] #13731: DeriveFunctor and friends don't understand type families

[GHC]

#13731: DeriveFunctor and friends don't understand type families
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        Reporter:  spacekitteh       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 What would you rather the error message say? I do think the phrase "the
 type variable" is unfortunate, as it should rather be "the //last// type
 variable" (since there can be more than one). But other than that, it's
 pretty spot-on: if you're using the last type variable within another
 type, it must be a data type, and `ExtensionType` is not a data type.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13731#comment:2>
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