You've clearly never bought and sold stock in two different lots in the US before. Brokerages report cost-basis to the IRS, this basis will be calculated based

You've clearly never bought and sold stock in two different lots in the US before. Brokerages report cost-basis to the IRS, this basis will be calculated based on your selection when you initiate the trade. You can't start talking about other capital gains in the same sentence as regulated securities as the reporting requirements are going to be different.