boost::variant with reference contend, not able to assign a value

I have a incomplete type

Struct A { char x; B b; }

Struct B { int x; A a; }

and a boost::variant containing an B element. There are three possibilitys to declare my variant:

typedef boost::variant<B, char, int> vari // this is wrong because of the incomplete type typedef boost::variant<B*, char, int> vari // this works

typedef boost::variant<B&, char, int> vari // this works also

but now i have the following funktion:

void setVari(vari& v){ int x; v = x; }

This function does only work if i use the second method to declare my variant. If i use the refrence contend, boost throws template argument deduction/substituition faild compiler errors.

Is there a possibility to make this function work, if i use a refrence instead of a pointer?