What's wrong with std.variant.Variant?

[Mathias LANG]

On Saturday, 13 June 2020 at 19:10:04 UTC, Andrei Alexandrescu 
wrote:
> I was curious about collecting a list of grievances about 
> Variant. It's the oldest piece of generic code in std, and it 
> predates a lot of good language additions.
>
> So what's wrong with Variant? One thing I collected from a 
> coworker is that it doesn't work with Windows DLLs, because in 
> turn typeof() comparison does not work across Windows DLLs.
>
> What are other problems with it?

I tried to use it a while ago and figured rolling out my own 
would be better (https://github.com/Geod24/minivariant). I've 
seen other (and probably better) implementations out there, but I 
mostly use this one because I'm familiar with the tradeoff.

I think most people want a tagged union over a generic variant 
type, however things are built the other way around: the tagged 
union is built on the generic variant type. Building a typed 
union over a type that relies on type erasure means Algebraic has 
to reimplement all the conversions rules that would normally be 
done by the compiler, e.g. assigning `immutable(uint)` to  a 
`ulong` should just work, but it does not.