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