[GHC] #11525: Using a dummy typechecker plugin causes an ambiguity check error

[GHC]

#11525: Using a dummy typechecker plugin causes an ambiguity check error
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        Reporter:  jme               |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |             Keywords:
      Resolution:                    |  UndecidableSuperClasses, plugin
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Thank you Adam.  I'm deeply swamped all this week and most of next. Could
 you articulate in a bit more detail, starting from zero, what the problem
 is, and why zonking `CDictCan` differently would help?  Then I'll try to
 respond.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11525#comment:5>
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