Error(s):

source_file.cpp:14:16: error: no matching function for call to 'f'
    void g() { f(0); } // error; works if void f() { ... } is commented out above
               ^
source_file.cpp:12:10: note: candidate function not viable: requires 0 arguments, but 1 was provided
    void f() { std::cout << "B::f()\n"; }
         ^
source_file.cpp:19:5: error: too many arguments to function call, expected 0, have 1; did you mean 'A::f'?
    B::f(0); // error; also works if void f() { ... } in B is commented out
    ^
    A::f
source_file.cpp:6:10: note: 'A::f' declared here
    void f(int) { std::cout << "A::f(int)\n"; }
         ^
2 errors generated.