[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 simonpj):

 I think Max was asking for something less fundamental.  He just wanted the
 `deriving` clause on the original instance decl to infer an instance
 context of `Eq (Associated a)`.  That's usually not allowed for `deriving`
 clauses but he argues that in the context of a class instance decl, it
 should.

 So it's just to do with the relatively narrow question of "what context
 does one infer for a deriving clause?".

 Since the standalone deriving clause is available, I'm inclined to say
 "use the standalone route".  It's just clearer.  But I'm not terribly
 bothered.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4815#comment:11>
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