Unnamed class/typename in template arguments

I was looking through the documentation of SFINAE and there was this template declaration:

template<typename SomeType>
struct inner_type { typedef typename SomeType::type type; };
template <
    class T,
    class = typename T::type,            // SFINAE failure if T has no member type
    class U = typename inner_type<T>::type // hard error if T has no member type
                                          // (guaranteed to not occur as of C++14)
> void foo(int) {}

Specifically, I'm asking about class = typename T::type. What's the point of declaring an unnamed class?

Because of the comment I thought that this will result in a compiler error when T doesn't have a member type, but that isn't the case, as foo<int, int, int>(0); compiles fine.

On the other hand

template<class T, typename = std::enable_if_t<std::is_unsigned<T>::value>>
void foo(T t) {}

doesn't compile if T is signed, and compiles if T is unsigned.

What am I missing here?