[GHC] #12600: Overloaded method causes insufficient specialization

[GHC]

#12600: Overloaded method causes insufficient specialization
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        Reporter:  akio              |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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 nfrisby):

 With GHC 7.10.2, adding another method to the class results in `foo` being
 as specialized as expected. I therefore suspect this ticket is due to the
 Specialise pass interacting poorly with casts (dictionaries for classes
 with one method and no superclasses are implemented as a newtype).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12600#comment:3>
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