Possible bug in RVO?

[Yuxuan Shui via Digitalmars-d-learn]

On Monday, 4 April 2016 at 03:28:01 UTC, Yuxuan Shui wrote:
> I have encountered a weird bug.
>
> I defined a Set class, which has a opBinary!"-". And somehow 
> this:
>
> auto tmp = set_a-set_b;
>
> produces different results as this:
>
> set_a = set_a-set_b;
>
> the latter will produce an empty set.
>
> I tried to reduce the source code to get a test case. But this 
> problem just goes away after removing some code.
>
> Any ideas what I could have done wrong?

A slightly more reduced test case:

struct Set {
	void insert(ulong v) {
		aa[v] = true;
	}
	@disable this(this);
	bool[ulong] aa;
}
auto clobber(ref Set x, ref Set o) {
	Set ret;
	ret.aa = x.aa;
	return ret;
}
struct XX {
	Set a, b, tmp;
	this(int n) {
		a.insert(1);
		//a.aa[1] = true;  <--- Swap above line with this doesn't 
trigger the bug
		tmp = a.clobber(b);
		a = a.clobber(b);
	}
}
void main(){
	import std.stdio;
	XX xx = XX(0);
	writeln(xx.a.aa.length);
	writeln(xx.tmp.aa.length);
}