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?