Why can't I template overload?

Why is it that this compiles:

class Testable {
public:
    template <bool flag>
    typename std::enable_if<flag>::type
    void foo() { cout << "Yay" << endl; }

    template <bool flag>
    typename std::enable_if<!flag>::type
    void foo() { cout << "Nay" << endl; }
};

But not if I define both foos using default types like so:

    template <bool flag, typename = typename std::enable_if<flag>::type>
    void foo() { cout << "Yay" << endl; } // (A)

    template <bool flag, typename = typename std::enable_if<!flag>::type>
    void foo() { cout << "Nay" << endl; } // (B)

I get this error (first line pointing to the definition of (B), second one pointing to (A)):

error: 'template<bool flag, class> void Testable::foo()' cannot be overloaded
error: with 'template<bool flag, class>> void Testable::foo()'