Understanding opAssign and 'alias this'

[Kozzi]

On Sunday, 13 October 2013 at 21:37:31 UTC, Maurice wrote:
> 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-

It is a bug from my POV