Out of curiosity I'm guessing it was left out of the C++11 standard do to ambiguous naming as functions/variables would live in an unnamed inner scope?

In the anonymous case:

void f() {
	union {
		struct { int x, y; };
		struct { int x, y; };
	} u;
}

In the not so anonymous case its harder to come up with the same example, but there are similar issues.

I should note that the only places I've actually seen people use anonymous unions/structures was usually almost always a hack that could have been avoided.

Out of curiosity I'm guessing it was left out of the C++11 standard do to ambiguous naming as functions/variables would live in an unnamed inner scope?

In the anonymous case:

void f() {
    union {
        struct { int x, y; };
        struct { int x, y; };
    } u;
}

In the not so anonymous case its harder to come up with the same example, but there are similar issues.