More on immutable

[bearophile]

I am trying to learn more about the usage of immutable. This is a little D2 
program:


immutable class S {
    static int x;
    static this() {
        S.x = 10;
    }
}
void main() {}


The compiler outputs:
test.d(3): Error: function test2.S._staticCtor1 without 'this' cannot be 
const/immutable

Can you tell me what's wrong or how I can fix it?

Bye and thank you,
bearophile