Variants and pattern matching implementation

[David B. Held]

Reiner Pope wrote:
> [...]
> 2. Lack of types for templates parameters.
> It would really be nice to express what *kind* of template parameters 
> you want in more detail (this specifically refers to alias parameters, 
> and their tuple counterparts). I know of at least three specific times 
> in writing the variant that I forgot/misunderstood the type the 
> parameter was -- I can't help feeling that some kind of Concepts idea 
> for template parameters would be nice.
> [...]

This is my favorite hobby-horse, so if you could elaborate on this with 
some specific examples, that would be great. ;)

Dave