Can't define function member class of template class that returns the member class inside the source file (cpp)

Question

I have a class and a nested class:

template<class T>
class foo
{
public:
    struct small_foo
    {
        small_foo& operator=(const small_foo& other);
    };
};

and a soruce file that looks like that:

#include "foo.h"

template<class T>
foo<T>::small_foo& foo<T>::small_foo::operator=(const small_foo& other)
{
    //Some code
    return *this;
}

While im compiling (static library) I'm getting 3 errors and a warning:

warning C4346: :small_foo": name is not a type

error C2061: syntax error : identifier 'small_foo'

error C2143: syntax error : missing ';' before '{'

error C2447: '{' : missing function header (old-style formal list?)

EDIT:

The problem was that the compiler couldn't tell what foo<T>::small_foo& in foo<T>::small_foo& foo<T>::small_foo::operator=(const small_foo& other) was. The solution is to add typename keyword in front of it so it looks like that:

template<class T>
typename foo<T>::small_foo& foo<T>::small_foo::operator=(const small_foo& other)
{
    //Some code
    return *this;
}