Scenario:
1. I buy 1 ETH for 1 USD when rates are 1 ETH/USD, 2 ETH/BTC.
2. I trade 1 ETH for 1 BTC when rates are 2 ETH/USD, 1 ETH/BTC.
Option 1: I’ve incurred a loss, because the sale is reported as first a sale of ETH for USD, then a purchase of BTC at whatever the cost basis between BTC/USD.
Option 2: I’ve incurred a gain, because I traded directly from ETH to BTC, and the relative value between ETH and BTC has gone up.