How to initialize a associative array?

[Yuxuan Shui via Digitalmars-d-learn]

On Saturday, 24 December 2016 at 00:55:01 UTC, Yuxuan Shui wrote:
> 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?

This example here: https://dlang.org/spec/hash-map.html, doesn't 
work either.