2894 – abstract classes sometimes allow non-abstract bodyless functions

This compiles and links
abstract class C { void foo(); }
void main() {}
while this doesn't
interface I { void foo(); }
abstract class C : I { void foo(); }
void main() {}
with an undefined reference to `_D7abstfun1C3fooMFZv'.
Since methods in abstract classes may have bodies and being allowed to implement a member function elsewhere than the class definition is a goal (see resolution of bug 1289), the first case should also require a definition for foo. This is also what I would expect from reading the spec on abstract functions and classes.

That the first example links is understandable - because C.foo is never used, the compiler doesn't create any references to it.
That the second example fails is semi-understandable - presumably the compiler creates a vtbl for C's implementation of I, but the linker cannot resolve it because no implementation of C.foo() has been linked in. However, it does seem that DMD could be better at DCE.
Less understandable is that this fails with the same error:
----------
abstract class C { void foo(); }
class D : C {}
void main() { }
----------
It was my understanding as well that a function in an abstract class is automatically abstract if no body is given. But it appears now that it's the same as in a non-abstract - if the function isn't declared abstract, it references an externally-defined function. Still, the spec probably needs to be clearer on the issue.

Commit pushed to https://github.com/D-Programming-Language/d-programming-language.org
https://github.com/D-Programming-Language/d-programming-language.org/commit/341a8f7233d74c9313625290eff8af57af3c2d2a
fix Issue 2894 - abstract classes sometimes allow non-abstract bodyless functions