Error(s):

source_file.cpp:14:20: error: call to 'f' is ambiguous
        void g() { f(); }
                   ^
source_file.cpp:6:10: note: candidate function
    void f() { std::cout << "A::f()\n"; }
         ^
source_file.cpp:9:14: note: candidate function
        void f() { std::cout << "A::B::f()\n"; }
             ^
1 error generated.