class = X is just an anonymous template type parameter with default value X, just like

    auto foo(int = 0);

declares a function with a int parameter with default value 0. In this case, it's just a conventional place to stick an expression in the declaration to trigger SFINAE. If you don't need SFINAE, you can also just place the condition in a static_assert in the definition

    template <class T>
    auto foo(T&&) {
        static_assert(!is_reference<T>::value, "call me with an r-value");
        ...
    }