Empty field doesn't exist for arrays, right?

[Andrej Mitrovic]

module test;

struct MyArray(T)
{
    private T[] data;
    
    bool opCast(T)() if (is(T == bool))
    {
        return !data.empty;
    }
}

void main()
{
    auto foo = MyArray!(int)();
    auto state = foo ? true : false;
}

test.d(13): Error: undefined identifier module test.empty
test.d(20): Error: template instance test.MyArray!(int).MyArray.opCast!(bool) 
error instantiating

This is straight from the book. Did .empty exist for arrays before? Perhaps 
this was just a typo in the book, and it was supposed to be:

    bool opCast(T)() if (is(T == bool))
    {
        return data.length != 0;
    }

Also, that error message *really* needs to improve. It's not module 'test' 
which is missing the method, it's 'data'. This is one of the most confusing 
error messages that I know of and it pops up all the time.