为什么在显式实例化类模板之前需要显式实例化外部类模板

也许是因为这样的原因:

template <typename T>
struct foo
{
    template <typename U>
    struct bar
    {
        typedef U type;
    };
};

template <typename T> 
struct foo<T>::bar<int> // imaginary
{
    typedef void type;
};

template <>
struct foo<float>
{
    template <typename U>
    struct bar
    {
        typedef U* type;
    };
};

// is it void [foo<T>::bar<int>] or 
// int* [foo<float>::bar<U>]?
typedef foo<float>::bar<int>::type ambiguous;