[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):

 What you want here has a number of problems.

 1.) It conflicts with `IncoherentInstances` by randomly changing answers
 based on inlining.

 2.) It doesn't really interact well with the notion of typechecking
 producing a witness that holds the elaborated code and just using the
 witness.

 3.) We actually '''have''' to take the "nearest" instance when you bind
 instances in local scope. Why? `ImplicitParams` unfortunately exist.

 If you have `(?x :: Int)` in scope

 and you come across `let ?x = 12 :: Int in ...` you need to switch to the
 new witness of `(?x :: Int)`.

 The same should work if you open a `Dict (?x :: Int)`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9701#comment:3>
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