Function taking nested template class of a template class as parameter

Can someone explain to me why this does not compile and how should I write it correctly?

#include <iostream>

using namespace std;

template <typename T>
struct Foo
{
    template <typename U>
    struct Bar
    {
    };
};

template <typename T, typename U>
void func(Foo<T>::Bar<U>& lhs)
{
    cout << "working" << endl;
}

int main()
{
    Foo<int>::Bar<char> a;
    func(a);

    return 0;
}

I get the following compiler error:

main.cpp:23:19: error: variable or field ‘func’ declared void  
void func(Foo<T>::Bar<U>& lhs)