Abstract functions in child classes

[Adam]

Ok, starting to feel like I'm missing something obvious...

The abstract keyword in the language reference states:
"Functions declared as abstract can still have function bodies. This
is so that even though they must be overridden, they can still
provide �base class functionality.�"

So, "they must be overridden." Does the compiler do *anything* to
verify this for a child class?

This compiles:

    import std.stdio;

    public abstract class Parent {
        public void hasDefinition() {
            writeln("I have a definition");
        }

        public abstract void noDefinition();
    }

    public class Child : Parent {
        public void unRelated() {
            writeln("Unrelated");
        }
    }

    void main() {
        Child child;
    }

However, if I change main() to:

    void main() {
        Parent instance = new Child();
    }

I get "cannot create instance of abstract class Child | function
noDefinition is abstract"

Why is a reference / use of child in the context of a parent
required just to validate that the class is a valid extension of the
parent? More to the point, why does the first case even compile?