>>61
G++の3.4.6, 4.0.3は、

foo.c: In function `int main()':
foo.c:4: error: use of `f' is ambiguous
foo.c:3: error: first declared as `int f()' here
foo.c:1: error: also declared as `struct N::f' here
foo.c:4: error: `f' was not declared in this scope