Why is this pure?

[Shachar via Digitalmars-d-learn]

The following program compiles, and does what you'd expect:
struct A {
    int a;
}

pure int func( ref A a )
{
    return a.a += 3;
}

As far as I can tell, however, it shouldn't. I don't see how or 
why func can possibly be considered pure, as it changes a state 
external to the function.

What am I missing? Or is this just a compiler bug?

Shachar