template <typename T>
struct A
{
    void foo(int);
};

template <>
void A<int>::foo(int)
{
}

template <typename> struct C {};

template <typename T>
struct A
{
    void foo(int);
};

template <typename T>
void A<C<T> >::foo(int)
{
}

test.cpp:10:23: error: invalid use of incomplete type 'struct A<C<T> >'
test.cpp:4:8: error: declaration of 'struct A<C<T> >'

template <typename T>
struct A
{
    template <typename U>
    void foo(U);
};

template <>
template <typename U>
void A<int>::foo(U)
{
}

template <typename> struct C {};

template <typename T>
struct A
{
    template <typename U>
    void foo(U);
};

template <typename T>
template <typename U>
void A<C<T> >::foo(U)
{
}