Why can't structs be derived from?

[Jens]

Jesse Phillips wrote:
> Jens Wrote:
>
>> OK, silly me. I used a wrong example. I really did want to know about
>> non-polymorphic composition from structs via derivation. Sorry for
>> the confusion.
>>
>> struct point
>> {
>>     int x;
>>     int y;
>> };
>>
>> struct point3d: point
>> {
>>     int z;
>> };
>
> You already got your answer 'alias this' which you have dismissed as
> ugly. You wanted composition and I think it provides a very clean way
> to describe composition, especially since it looks nothing like how
> inheritance of classes which have polymorphic behavior.

Inheritance and polymorphic behavior can be (should be IMO) orthogonal.

> Also note the
> intent is to allow for multiple 'alias this' declarations, but hasn't
> yet been implemented.
>
> struct point
> {
>     int x;
>     int y;
> };
>
> struct point3d
> {
>     point p; // I own a point
>     alias this p;  // I am a composition of a point p.
>     int z;
> };

I didn't ask how to do composition in D. I asked why composition cannot 
be done via derivation, i.e., the reasoning behind the language design 
choice. A design faux paus IMO.