If the security sold is a MLP it is not so simple because the taxpayer must adjust the cost basis that the broker reports using data provided by the MLP in its K-1. This is because some, or all, of the funds paid out as dividends are treated as a return of capital because the large amount of asset depreciation creates an accounting loss but has no effect on the MLP's cash flows.
The taxpayer has to determine the cost basis of each lot of shares sold because the K-1 provides the adjustment for each such sale. Wash sales further complicate this because of the need to track where the disallowed losses get allocated. Making things worse, you can't trust the broker to do this correctly.
