Make 'abstract' mandatory if the class is intended to be abstract, absence of 'abstract' means "not abstract"

[Ary Borenszweig]

A class can either be abstract or not abstract. Currently in D, if you 
don't mark a class as abstract, it can still be it if it contains an 
abstract method:

class Foo {
	
	abstract void someAbstract();
	
	void nonAbstract() {
	}
	
}

When designing a class, you have in mind whether the class is going to 
be abstract or not. If it's not going to be abstract, you want the 
compiler to help you by telling you "You made a mistake. This class is 
still abstract because you didn't implement method foo".

So I want to extend Foo with a class Bar, but I want Bar to be not abstract.

class Bar : Foo {
}

I compile, and it gives no error, of course. But I want there to be an 
error there. The only way I can get an error is by making a dummy 
function that instantiates Bar:

void blah() {
	Bar bar = new Bar();
}

main.d(14): Error: cannot create instance of abstract class Bar
main.d(14): Error: function someAbstract is abstract

The problems with this appraoch are two:
  - You have to make a dummy function to check whether you implemented 
Bar correctly.
  - You get two errors for each instantiation of Bar, if it's abstract 
(ugly).

Why not make "abstract" mandatory for a class if it's intended to be 
abstract, and the absence of "abstract" to mean "not abstract"? Java 
works this way, and I think it is for the reasons I mentioned.

Another advantage is that just by seeing the start of class definition 
you can tell whether a class is abstract or not. You don't have to see 
if any method is marked as abstract, or go to the superclasses to see if 
there is a method that is still not implemented.

(also, it would be nice if the compiler could tell you all the methods 
that still need an implementation, rather than just one)