Null references redux + Cyclone

[Nick Sabalausky]

"bearophile" <bearophileHUGS at lycos.com> wrote in message 
news:h9qo0l$750$1 at digitalmars.com...
>
> But beside normal C unions that I don't want to remove from C, it can be 
> also useful to have safe automatic tagged unions of Cyclone. They are 
> safer and give just a little less performance compared to C unions. In D 
> they may be denoted with "record" or "tunion" or just "safe union" to save 
> keywords. They always contain an invisible tag (that can be read with a 
> special built-in union method, like Unioname.tagcheck). Such "safe unions" 
> may even become the only ones allowed in SafeD modules!
>
> The following is from Cyclone docs:
> <<
> The C Standard says that if you read out any member of a union other than 
> the last one written, the result is undefined.
> To avoid this problem, Cyclone provides a built-in form of tagged union 
> and always ensures that the tag is correlated with the last member written 
> in the union. In particular, whenever a tagged union member is updated, 
> the compiler inserts code to update the tag associated with the union. 
> Whenever a member is read, the tag is consulted to ensure that the member 
> was the last one written. If not, an exception is thrown.
>

Sounds like a variant to me, but just without automatic conversion. Or am I 
misunderstanding?