Understanding opAssign and 'alias this'

[Maurice]

Hello,

Can anybody explain my what is happening here?
enum xxx = true;

struct A {
        static if (xxx) ref A opAssign(ref const A a) { return this; }
        ref A opAssign(int v) { return this; }
}

struct B {
        A a;
        alias a this;
}

void main() {
        A a;
        B b;
        a = b; // [1]
        b = a; // [2]
}

When xxx is false:
[1] Gives an error
[2] Compiles fine

When xxx is true:
[1] Compiles fine
[2] Gives an error

What exactly is happening here? When xxx is false, what does 'b = 
a' do that is wrong when xxx is true?

I can't find any clear documentation about opAssign and its 
'default' implementation.

Thanks!

-Maurice-