Why is typename required in front of template function? [duplicate]

I'm using an example from Stroustrup C++ 4th Ed Page 692. Does anyone know why typename must come before Iter::value_type? Specifically in relation to the second mean function, why is typename not required for this template function?

I have read through Stroustrup's book sequentially and am in the templates section. I do not recall reading about the reasoning behind this.

Thanks for your guidance!

template<typename Iter>
typename Iter::value_type mean(Iter first, Iter last);

template<typename T>
T mean(T*,T*);

void f(vector<int>& v, int* p, int n)
{
    auto x = mean(v.begin(),v.end());
    auto y = mean(p,p+n);
}