Generically call a function on Variant's payload?

[Nick Sabalausky (Abscissa)]

Suppose I've wrapped a Variant in a struct/class which ensures the 
Variant *only* ever contains types which satisfy a particular constraint 
(for example: isInputRange, or hasLength, etc...).

Is there a way to call a function (ex: popFront) on the Variant, 
*without* knowing ahead of time all the possible static types it might 
might contain?