The difference is that (i) is, in a sense, generative - because you still have to declare a type to be an instance of CD even if it is one of both C and D.

That is not only inconvenient it can even create problems, for modular program development, as class instances always cross module boundaries [which is a wart]. So if there are two different modules in your project needing a type to be an instance of CD, you need to find a single place where to put that instance declaration.

Frankly, as long as there is no way to limit the scope of an instance it would probably even make sense to treat method-less classes as class synonyms anyway, i.e. ones that do not require instances. Though ways of limiting the scope of class instances is probably a change quite a few people would like to see, so such a change would inhibit a move in that direction in the future.

Stefan Kahrs