[GHC] #12814: Should GND infer an instance context when deriving method-free classes?

[GHC]

#12814: Should GND infer an instance context when deriving method-free classes?
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11369, #12810    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I suppose so. But it would also seem possible to skip figuring out, a
 priori, what constraints should be there. Can't we just type-check the
 generated code and then posit whatever constraints are necessary for that
 code to be accepted? That's what we do for `stock` instances, no?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12814#comment:5>
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