Re: class inside of template puzzler

I've reached a bit of code that I'm failing to understand. The error
generated by g++ 4.* is below each attempt to get the code working.
The simple distilled down version is below:

#include <map>

template<class P1>
struct T
{
   struct T2
    {
        T2() {}
    };

    T()
    {
      std::map<std::string, T2> m;

      std::map<std::string, T2>::iterator itr1 = m.begin();
        //error: expected `;' before 'itr'

      std::map<std::string, T::T2>::iterator itr2 = m.begin();
        //type/value mismatch at argument 2 in template parameter list
for 'template<class _Key, class _Tp, class _Compare, class _Alloc>
class std::map'
        // expected a type, got 'T<P1>::T2'
        // template argument 4 is invalid
        // expected initializer before 'itr2'

      std::map<std::string, T<P1>::T2 >::iterator itr3 = m.begin();
        //Same error as above

      std::map<std::string, T<int>::T2 >::iterator itr4 = m.begin();
        // Compiles, but not really useful

      typename std::map<std::string, T2>::iterator itr5 = m.begin();
        // Works as expected
    }
};

int main() {}

I understand that typename is meant to be used in cases where the
compiler thinks there is an ambiguity and it does not know if you are
referring to a type or a variable. However, I don't understand why it
is is needed in the case above.

Case in point, the declaration of "m" does not need "typename."