From the "currency" point of view - exchange rate (FX) gains/losses technically only apply to officially recognized foreign currencies.
So that is an interesting can of worms. If I buy bitcoin at price point X, it goes up to X+Y, but instead of selling it I use that Y to pay for something - was that Y a taxable gain?
https://www.irs.gov/uac/newsroom/irs-virtual-currency-guidan...
Which links to: https://www.irs.gov/pub/irs-drop/n-14-21.pdf
TL;DR - it's not currency, it's property, and as taxed as such in all ways (including recognizing gain/loss).
Specifically re: your question:
> Q-6: Does a taxpayer have gain or loss upon an exchange of virtual currency for other property?
> A-6: Yes. If the fair market value of property received in exchange for virtual currency exceeds the taxpayer’s adjusted basis of the virtual currency, the taxpayer has taxable gain.
IANAL, etc.
Edit: Well, for some definition of straightforward :)
[1] - https://www.irs.gov/uac/newsroom/irs-virtual-currency-guidan...
Even more fun, if you mine back in the old days when difficulty was 3-digits, and then sell years later, they can be a long term capital gain.