How to initialize a associative array?

[Yuxuan Shui via Digitalmars-d-learn]

I tried this:

     immutable int[char] xx = ['Q':0, 'B':1, 'N':2, 'R':3, 'P':4];

And got a "non-constant expression" error (with or without 
'immutable').

What's the correct way?