You would report it
in 2017.
Be sure that the non-deductible
IRA contribution is made for the 2017 tax year and not 2016. The second step in the process, changing from
the Traditional IRA to the Roth IRA, will generate and Form 1099-R that you
will not receive until early 2018.
Therefore, it will be reported on your 2017 tax return.
