*final* class member (D2)

[Denton Cockburn]

How do I do this?  I'm trying to create a class member here that needs to
be initialized in the constructor, but cannot be changed later.

class A
{
   (const doesn't work) real delegate() dg; // this is that
   function;

   this(real delegate() dg) { this.dg = dg; }
}

class B
{
   real foo() { return 3.5; }
}

class C
{
   real foo() { return 4.5; }
}

void main()
{
   auto b = new B;
   auto a = new A(&b.foo);
   auto c = new C;
   a.fitness = &c.foo; // I want this to be an error
}