Why does std.variant not have a tag?

[Robert Jacques]

On Sunday, 4 November 2012 at 22:33:46 UTC, Alex Rønne Petersen
wrote:
> On 05-11-2012 00:31, evansl wrote:
>>   http://dlang.org/phobos/std_variant.html
>>
>> says:
>>
>>  This module implements a discriminated union type (a.k.a. 
>> tagged union,
>> algebraic type).
>>
>> Yet, the wiki page:
>>
>>   http://en.wikipedia.org/wiki/Tagged_union
>>
>> says:
>>
>>   a tag field explicitly indicates which one is in use.
>>
>> and I don't see any indication of a tag field in the 
>> std_variant.html
>> page.  Another wiki reference:
>>
>>   http://en.wikipedia.org/wiki/Disjoint_union
>>
>> is more explicit because it pairs the tag with the value:
>>
>>   (x,i)
>>
>> where x is the value and i is the tag.
>>
>> One reason for an explicit tag is that the bounded types may 
>> contain
>> the same type twice.  This has lead to problems in 
>> boost::variant as
>> evidenced by the post:
>>
>>   
>> http://article.gmane.org/gmane.comp.parsers.spirit.general/17118
>>
>> In addition, both variant and tuple have a common part, a 
>> metafunction
>> mapping from a tag to a type; hence, this same common part 
>> could be used
>> to implement both tuple and a tagged variant.
>>
>> A variant which actually contained a tag field I think would 
>> be more
>> general in that it would allow duplicate types among the 
>> bounded types
>> just as a tuple's bounded types can contain duplicate types.
>>
>> -regards,
>> Larry
>>
>
> Yes, this is a big problem with the current std.variant 
> implementation (among other problems such as no recursive 
> variants....). The best std.variant can offer right now is the 
> 'type' property to identify what's stored in it.
>
> std.variant is, unfortunately, not very useful if you want the 
> semantics of variants in ML-style languages.

I've been working on an update to std.variant whose formal
submission has been held up by a PhD thesis and some health
issues, although I'm currently (slowly) doing a code
review/cleanup of it in the hope of finally submitting it. ( Old
code: https://jshare.johnshopkins.edu/rjacque2/public_html/ )
Anyways, my implementation has an internal 'tag' as does the
current implementation, IIRC. However, as the tag is a
meaningless random number, instead .type is provide access to the
meaningful typeinfo object of that class. And .type provides most
(all?) of the functionality of an int id:

auto var = Variant(5);
if(var.type == typeid(int)) {
      // Do something...
} else if(var.type == typeid(string)) {
     // Do something else...
}

But I am missing something as I found that the linked post wasn't
clear what the exact issue was, only that there was an issue. If
someone would like to clarify the problem (or any other with
Variant) it would be appreciated.