Incorrect handling of Self type in abstract parent

[bendmorris]

• Kit version: dev
• OS/Platform: all

Describe the problem. What did you see? What did you expect to see?

• A method which returns Self, when fallen through from an abstract, should return the abstract type, not the parent type.

If this is a code issue, provide a minimal code example:

struct Parent {
    public var x: Int;
    public var y: Int;

    public function copy(): Self {
        return struct Self {x: this.x, y: this.y};
    }
}

abstract Child: Parent;

function main() {
    var x: Child = struct Child {x: 1, y: 1};
    var y: Child = x.copy();
}

----------------------------------------
Error: ./test.kit:14: A variable's initial value must match the variable's type:

  @./test.kit:14:20-25
      14        var y: Child = x.copy();
                               ^^^^^^

    Expected type:  test.Child
      Actual type:  test.Parent