Consider the following program:
m>

enum X { a = 1 };
enum Y { b = 1 } __attribute__((packed));

int main()
{
  using namespace std;
  cout << "default:        " << sizeof(enum X) << endl;
  cout << "with attribute: " << sizeof(enum Y) << endl;
}

The C++ compiler accepts this.  However, the attribute on enum Y is ignored.  As running this program 
shows, the size of enum Y is the same as the size of enum X.

Note that the test suite contains a test to make sure that an attribute in this position is accepted.  
(parse/parse3.C) We just failed to make sure that the attribute really did anything.