What would break if class was merged with struct

[Ola Fosheim Grøstad via Digitalmars-d]

On Sunday, 28 May 2017 at 09:09:39 UTC, Iain Buclaw wrote:
> However, for extern(D) classes, there's actually nothing 
> stopping a D compiler implementer from making class types more 
> memory efficient, an optimizing compiler could decide to 
> re-arrange class fields as:
>
>     class Something
>     {
>         long b;
>         int a;
>         char c;
>     }

Yes, but you could do that for structs as well, after static 
analysis establishing that it no code in this specific program is 
making assumptions about layout.

Or you could have an attribute that says that that field X should 
be at byte-offset N, but all other fields can move.

> As a side note, when I talked about D at the GNU Hackers 
> Meeting a number of years back, when I said that structs and 
> classes are two distinct types - one being POD-only, the other 
> being a reference that comes with the bells and whistles of OOP 
> - I actually got a small cheer.  This gives me a very strong 
> impression of what C++ programmers think of their own situation 
> regarding struct vs class, and that we should not be so eager 
> to follow their design.

Did they explain why? Many other languages provide a single 
construct with far more wide-ranging features than D/C++.

For generic programming it is generally better to have fewer 
constructs, but you might need a way convenient way to express 
constraints (i.e. C++ concepts).