Associative arrays give compile error

[Bob Cowdery]

 I can't seem to get any sense out of associative arrays. Even the
simplest definition won't compile so I must be doing something daft.

int[string] aa = ["hello":42];

Error: non-constant expression ["hello":42]

What exactly is not constant about this. The example is straight out the
book. Using D 2.0.

bob