What would break if class was merged with struct

[Ola Fosheim Grøstad via Digitalmars-d]

On Saturday, 27 May 2017 at 19:01:12 UTC, Moritz Maxeiner wrote:
> 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.

Does the language spec say anything about the size of class 
references?

> 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.

Is it specified?

The common implementation uses a GC, but does it say that an 
implementation cannot use a reference counted smart pointer or a 
fat pointer instead?

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

That really depends on the definition of the language.