[GHC] #9701: GADTs not specialized properly

[GHC]

#9701: GADTs not specialized properly
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              Reporter:  dfeuer      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.9
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  Runtime     |       Blocked By:
  performance bug                    |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by ekmett):

 Reid Barton mentioned a fairly simple scheme of skimming the core looking
 for terms of type `C T` where you have an `instance C T` declaration in
 scope with no context, and replacing those terms with the dictionary for
 the witness.

 That could make cases like `Set`-carrying-a-dictionary-for-`Ord`
 optimizable for known instances.

 It'd be the most specific match, should win for `IncoherentInstances`,
 doesn't collide with the `(?x :: Int)` case.

 The trick then is selling an optimization for a style of code that
 currently doesn't get written.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9701#comment:4>
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