[GHC] #12791: Superclass methods could be more aggressively specialised.

[GHC]

#12791: Superclass methods could be more aggressively specialised.
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        Reporter:  mpickering        |                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 simonpj):

 Like I say, I could imagine a special case for the situation where the
 constraint is `C t1 .. tn` and it can be solved by a top-level instance
 that has no context.  It'd be ad-hoc but probably useful, and I agree that
 the "might use the wrong dictionary" thing is probably an edge case.

 Beyond that, as I show above, I don't think we can use top-level
 instances.

 What I don't know is whether this special case would solve the "ton of
 cases" that you have seen in your code.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12791#comment:8>
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