To avoid some linking errors

[bearophile]

This code compiles with no errors, and then later the linker 
gives a "Symbol Undefined":


abstract class A {
     public void foo();
}
class B : A {}
void main() {}


In this bug report I have asked for the compiler to give an error:
http://d.puremagic.com/issues/show_bug.cgi?id=5129


But Walter has answered it's not a bug:

> This is not a bug, as in another module there could be a class 
> C that derives
> from B and implements foo().
> 
> As documented, D accepts non-abstract functions with no body 
> declared as:
> 
>    void foo();
> 
> with the idea that the user will be supplying a body somewhere 
> else - perhaps
> even a C function or an assembler one. It's another way of 
> doing encapsulation
> by having an opaque implementation. In fact, it's used by the 
> TypeInfo's.


Stewart Gordon suggests:

> I think the underlying problem is that there's no mandatory 
> explicit notation
> for externally defined functions.


So isn't it better to require (similarly to annotations like 
"override") the programmer to write in B an "extern" or 
"abstract" or something similar to state that the implementation 
is elsewhere (and give a nice compilation error if it's missing)?


abstract class A {
     public void foo();
}
class B : A {
	extern foo;
}
void main() {}


Or:

abstract class A {
     public void foo();
}
class B : A {
	abstract foo;
}
void main() {}


Bye,
bearophile