Add virtual classes

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Finish implementing virtual classes based upon the current proposal.

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Imported From: https://issues.scala-lang.org/browse/SI-634?orig=1
Reporter: Geoffrey Alan Washburn (washburn)

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Geoffrey Alan Washburn (washburn) said:
Lots of problems with this at present. Maybe the visiting student this fall can come up with something.

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Geoffrey Alan Washburn (washburn) said:
Visiting student has materialized, so I assign it to him.

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@odersky said:
Milestone next_feature deleted

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@paulp said:
Given that a bunch of less ambitious tickets have been closed as requiring a SID, I'm closing this as requiring a SID. Or at least requiring something other than a limbo ticket.

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@soc said:
An example where things currently behave in a way which seems to violate a lot of reasonable expectations:

  class Foo {
    class Bar {
      def foo = 1
    }
  }
 
  class SubFoo extends Foo {
    class Bar {
      def foo = 42
    }
  }

// Let's create some instances:
  val foo = new Foo
  val fooBar = new foo.Bar
  fooBar.foo                                      //> res0: Int = 1
  // ok

  val subFoo = new SubFoo
  val subFooBar = new subFoo.Bar
  subFooBar.foo                                   //> res1: Int = 42
  // ok

  val superFoo: Foo = subFoo
  val superFooBar = new superFoo.Bar
  superFooBar.foo                                 //> res2: Int = 1
  // NOT OK!

The last result is scary, because the syntax looks like things are dynamically bound, but the method is called on the static type instead.

Mailing list: https://groups.google.com/d/topic/scala-language/vQYwywr0LL4/discussion

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@soc said:
I'm assigning this to me for now. I'll ping the person who wanted to work on virtual classes for Scala and see what's his status.
Otherwise, I'll look into it for 2.12 myself.

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@soc said:
Reopening, so that I don't forget to look into it.