Template function in template class can only be called if its signature exists outside the class

Question

I can compile and run this program, but I don't really understand why I can't compile it when I remove the

template <int W>
void bar();

part from it. Calling functions of Testclass directly from main() is no problem, however I cannot call the template function bar() from another template function, testfunction(), in my case.

When I attempt to compile without the above-mentioned two lines, gcc 4.7 stops with "error: invalid operands of types '' and 'int' to binary 'operator<'" and I don't understand why and I understand even less why I can compile and run the program with these two lines added. Clang however doesn't compile any of the two versions. Can anyone explain to me what's going on here?

#include <iostream>

template <int T>
class Testclass {
    public:
    void foo(void) {
        std::cout << T << std::endl;
    }

    template <int U>
    void bar(void) {
        std::cout << T << " " << U << std::endl;
    }
};

template <int W>
void bar();

template <int V>
void testfunction( void ) {
    Testclass<V> otherTestObj;
    otherTestObj.foo();
    otherTestObj.bar<4>();
}

int main() {
    Testclass<1> testobject;
    testobject.foo();
    testobject.bar<2>();
    testfunction<3>();

    return 0;
}