GCC compilation error when using template class method inside lambda expression

The following code fails to compile in gcc 4.6.3, while it compiles flawless in clang 3.1 (I provide two versions of the class DummyMath, both exhibit the same problem):

#include <iostream>
using namespace std;

//* // To swap versions, comment the first slash.

// ===============================================
// V1
// ===============================================
template <typename T>
class DummyMath
{
public:
    T sum(T a1, T a2);
    T mult(T a1, int n);
};

template <typename T>
T DummyMath<T>::sum(T a1, T a2)
{
    return a1 + a2;
}

template <typename T>
T DummyMath<T>::mult(T a1, int n)
{
    auto x2 = [this](T a1) -> T
    {
        return sum(a1, a1); // <------- gcc will say that "sum" was not declared in this scope! 
    };

    T result = 0;
    n = n/2;
    for(int i = 0; i < n; ++i)
        result += x2(a1);
    return result;
}
/*/
// ===============================================
// V2
// ===============================================
template <typename T>
class DummyMath
{
public:
    T sum(T a1, T a2)
    {
        return a1 + a2;
    }

    T mult(T a1, int n)
    {
        auto x2 = [this](T a1) -> T {
            return sum(a1, a1);
        };

        T result = 0;
        n = n/2;
        for(int i = 0; i < n; ++i)
            result += x2(a1);
        return result;
    }
};
//*/

int main()
{
    DummyMath<float> math;
    cout << math.mult(2.f, 4) << endl;

    return 0;
}

The error is:

main.cpp:25:20: error: ‘sum’ was not declared in this scope

Both versions (V1 and V2) of class DummyMath fail in gcc, and both succeed in clang. Is that a bug in GCC?

Thanks.