#include <boost/variant.hpp>

class A {};
class B {};
class C {};
class D
{
	operator C() const { return C(); }
};

typedef boost::variant< A, B > variant_type;

void f( variant_type const& ){}
void f( C const& ){}

int main()
{
	f( A() );
	f( B() );
	f( C() );
	f( D() ); // ambiguous call to overloaded function
}