immutable method not callable using argument types () - doesn't make sense

[Daniel Donnelly]

I have: [code]

     module A;
        module A;

        interface B {
           public:
                  immutable B dup();
        }

        class A : B {
           public:
                  this() {}
                  this(in char[] field) { this.field = field.dup; }
                  immutable A dup() { return new A(field); }
           private:
                  char[] field;
        }

        void main()
        {
                B f, g;
                f = new A;
                g = new A;
                
                f = g.dup;
        }
I get