Yes basis carries forward until distributed. However, I don't understand your latest comment. What modified AGI was within the income limit for a Roth IRA contribution? In your question you said that your 2016 MAGI for the purpose was above the limit. Are you referring to you 2018 MAGI being below the limit, allowing your excess Roth IRA contribution carried in from 2017 to be applied as part or all of your 2018 Roth IRA contribution?
Note that if you apply an excess traditional IRA contribution or, what seems to be in your case, an excess Roth IRA contribution as a contribution for a future year, TurboTax mishandles the basis calculation by failing to reduce your basis by the amount of excess contribution applied while simultaneously increasing the basis by the amount of the resulting nondeductible contribution.
