Error(s):

source_file.cpp: In member function ‘int D<T>::g()’:
source_file.cpp:15:14: error: there are no arguments to ‘f’ that depend on a template parameter, so a declaration of ‘f’ must be available [-fpermissive]
     return f();  // error: should be ‘this->f()’
              ^
source_file.cpp:15:14: note: (if you use ‘-fpermissive’, G++ will accept your code, but allowing the use of an undeclared name is deprecated)