[GHC] #11008: Difficulties around inferring exotic contexts

[GHC]

#11008: Difficulties around inferring exotic contexts
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      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 crockeea):

 Without knowing anything about the internals of GHC, it seems there are
 two ways to simplify instance contexts: either we "simplify and reject" if
 they are "exotic" (in the case of GND instances), or just "simplify and
 typecheck" (in the case of hand-written instances of any variety). So if
 an instance head matches (Eq (Foo r)), trigger the "simplify and
 typecheck" rather than "simplify and reject exotic constraints" behavior.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11008#comment:4>
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