Replacing class name with an alias

C++11 type alias is great and I use it a lot but recently I've been wondering why type aliasing doesn't work like such #include

template<typename T>
class Y
{
    public:
    Y(): t( T{} ) {}
    T getT() const;

    private:
    T t;
};

template< typename T>
using Foo = Y<T>;


template< typename T>
T Y<T>::getT() const { return t; } //FINE

/* NOT FINE!
template< typename T>
T Foo<T>::getT() const { return t; }
*/

int main()
{
    Y<int> f{};
    return 0;
}

Question: If a type is aliased, why can't we replace the type aliased with the new alias name?