Error(s):

source_file.cpp: In instantiation of ‘struct A<B<float> >’:
source_file.cpp:11:8:   required from ‘struct B<float>’
source_file.cpp:20:14:   required from here
source_file.cpp:7:10: error: no type named ‘value_type’ in ‘struct B<float>’
     void foo(typename B::value_type arg) {}
          ^
source_file.cpp: In instantiation of ‘B<T>::B() [with T = float]’:
source_file.cpp:20:14:   required from here
source_file.cpp:15:21: error: ‘foo’ is not a member of ‘A<B<float> >’
         A<B<T>>::foo(T());
                     ^