reimplementing an interface in a derived class

[kdevel]

On Friday, 4 January 2019 at 20:21:56 UTC, Steven Schveighoffer 
wrote:
>> missing in the source. But why is d a null reference in the 
>> first place?
>
> Because when you dynamically cast one object or interface to 
> another object or interface, and that result is not possible 
> (if you remove ",D" from the example you quoted, then neither A 
> nor B implement D), then the result is null.

I overlooked that Alex wrote

    class A

and not

    class A : D

which I came across in the examples in #10 and #11 of

    https://dlang.org/spec/interface.html

and to which I referred to in my OP.

> https://dlang.org/spec/expression.html#cast_expressions
>
> See parts 2 and 3.

    part 2
    "Any casting of a class reference to a derived class reference
    is done with a runtime check to make sure it really is a 
downcast.
    null is the result if it isn't."

Part 2 is about downcasting. Does that apply here? Due to the 
omission
of ": D" in Alex' example the cast is not a cast "to a derived 
class
reference" and hence this part does not apply. OTOH upcasting is 
also
not covered by part 2 and seems to be legal:

part 2 at its face value:
```part2.d
import std.stdio;

class A {
    void foo () { __FUNCTION__.writeln; }
}

class B : A {
    override void foo () { __FUNCTION__.writeln; }
}

void main ()
{
    auto x = new B;
    auto u = cast (A) x;
    typeof (u).stringof.writeln;
    u.foo;
}
```
$ ./part2
A
part2.B.foo