[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 RyanGlScott):

 Ah, the comparison to `-XDatatypeContexts` is definitely a compelling
 argument for the current behavior. I'll quit my job as devil's advocate,
 then.

 In that case, that cements my opinion that associated data family
 instances should be totally unrelated to the instance context of the
 parent instance, and moreover, that the bug is really just a duplicate of
 #11008 in disguise, as the fix for this issue would be improving GHC's
 ability to infer exotic constraints.

 Should we close it?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4815#comment:16>
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