[GHC] #4815: Instance constraints should be used when deriving on associated data types

[GHC]

#4815: Instance constraints should be used when deriving on associated data types
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        Reporter:  batterseapower    |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  6.12.3
      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 rwbarton):

 I would expect it to type check, because an associated family instance is
 sugar for an ordinary data family instance and I can't understand what it
 would mean to put a constraint on such an instance (or why I would want
 to; it feels like the old datatype contexts).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4815#comment:15>
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