From the forum discussion:
http://forum.dlang.org/thread/kjv4ae$3hp$1@digitalmars.com

This code should call void opAssign(S rhs) twice.

import std.stdio;
import std.string;

version = destructor_defined;

struct S
{
    int i;

    string info() const
    {
        return format("%s(%s)", &i, i);
    }

    void opAssign(S rhs)
    {
        writefln("%s from Rvalue %s", this.info(), rhs.info());
    }

    void opAssign(ref S rhs)
    {
        writefln("%s from Lvalue %s", this.info(), rhs.info());
    }

    version (destructor_defined)
    {
        ~this()
        {
            writefln("destroying %s", this.info());
        }
    }
}

S foo(int i)
{
    return S(i);
}

void main()
{
    auto s = S(1);

    // Assignment from two kinds of rvalues
    s = foo(2);  // [1]
    s = S(3);    // [2]
}

In [1], void opAssign(S rhs) is called correctly.
But in [2], void opAssign(ref S rhs) is wrongly called.