struct QString
{
    ref QString opOpAssign(string op : "+")(ref QString s)
    {
        return this;        
    }
}

void main()
{
    QString s;
    s += s; // fails
    s.opOpAssign!("+")(s); // compiles 
}

test.d(12): Error: variable test.QString.op only parameters or foreach declarations can be ref

Compiles if the return type is omitted or the operator is called explicitly.