Inheritance

[josh11b]

This proposal adds inheritance to classes, following this syntax:

// Abstract classes may not be instantiated, but
// may have abstract methods and be extended.
abstract class AbstractBaseClass {
  virtual fn CanBeOverridden[me: Self]() -> i32 {
    return me.data;
  }
  abstract fn PureVirtual[me: Self]() -> i32;
  fn Create(data: i32) -> partial Self {
    return {.data = data};
  }
  protected var data: i32;
}

// Classes are final by default
class FinalClass extends AbstractBaseClass {
  impl fn PureVirtual[me: Self]() -> i32 {
    return me.x;
  }
  fn Create(data: i32) -> Self {
    return {.base = AbstractBaseClass.Create(data), .x = 2 * data};
  }
  private var x: i32;
}

// Can be instantiated and extended
base class ExtensibleClass {
  // May optionally use partial types in factory functions
  protected fn CreateAsBase(data: i32) -> partial Self { ... }
  fn Create(data: i32) -> Self { ... }
}

[chandlerc]

Got a first pass over this. Mostly trying to clarify wording, generally very happy with the direction here. A few places with more substantial changes, but nothing that I think is changing the direction at all (and give a shout if I've said anything that gives a different impression).

[chandlerc]

I think this is basically good to go. I'd like to double check that others aren't interested in digging into it before it lands, and I'd like to officially close the blocking issues, but not seeing anything major left.

I suspect we may have some follow-up changes to tweak things, but I don't think that should hold up this version landing.

Couple of minor wording suggestions below, but none of these are really big issues, just me attempting to improve clarity.

[chandlerc]

Minor tweak below. LGTM with that, and 🎉🎉🎉 for inheritance landing!