What would break if class was merged with struct

[Moritz Maxeiner via Digitalmars-d]

On Saturday, 27 May 2017 at 18:42:02 UTC, Ola Fosheim Grøstad 
wrote:
> On Saturday, 27 May 2017 at 18:26:07 UTC, Moritz Maxeiner wrote:
>> mechanism, but you *cannot*  use a smart pointer as a 
>> replacement for a class instance, because the latter is normal 
>> pointer, and the former adds features on top of a pointer; 
>> they cannot be the same as a matter of definition, i.e. you 
>> cannot replace classes with smart pointer structs on the 
>> language level, because it removes features from the language.
>
> I am not sure what you mean here. As long as the "smart 
> pointer" is in a subtype relationship with the current "class 
> reference" then it can add features, yes?

An example, then:

---
module a;
class Foo {}

module b;
import a;
void bar(Foo foo) {}
---

Here, `bar`, takes a (pointer to a) class instance as parameter 
`foo`. `foo` is a single pointer, i.e. 8 bytes on a 64bit OS, 
with *no* special semantics.
You cannot replace the meaning of type `Foo` here with something 
that is not also exactly 8 bytes (on 64bit OS) with no special 
semantics, because that changes the semantics of `bar`. i.e. 
`Foo` must not be a smart pointer because a smart pointer 
necessarily introduces features (and thus changes the semantics).
And this is exactly what you were proposing in the sentence I 
replied to; you were proposing to essentially change the 
semantics of every use of a class instance in all D code by 
rewriting `Foo` into a smart pointer.

>
> The point of this thread was to have fewer language constructs,

And my reply was "if you want to do this, it must use a normal 
pointer, not a smart pointer, because that changes semantics". If 
you want a smart pointer, use one, but don't suddenly make 
something that isn't a smart pointer by language design change 
shape into a smart pointer.

> with more lowering, so that the resulting language would be a 
> super-language, that fully covers the original language (or 
> close to it).

Sure, but then type `Foo` in the above must remain a normal 
pointer, not become a smart pointer.