from 8606 instructions
Line 4
If you made contributions to traditional
IRAs for 2019 in 2019 and 2020 and you
have both deductible and nondeductible
contributions, you can choose to treat
the contributions made in 2019 first as
nondeductible contributions and then as
deductible contributions, or vice versa.
Example. You made contributions
for 2019 of $2,000 in May 2019 and
$2,000 in January 2020, of which
$3,000 are deductible and $1,000 are
nondeductible. You choose $1,000 of
your contribution in 2019 to be
nondeductible. You enter the $1,000 on
line 1, but not line 4, and it becomes
part of your basis for 2019.
Although the contributions to
traditional IRAs for 2019 that you made
from January 1, 2020, through April 15,
2020, can be treated as nondeductible,
they aren’t included in figuring the
nontaxable part of any distributions you
received in 2019.
this makes sense because if all n/d contributions were made after 12/31 any distributions would have had to come out of 2018 IRA's
lines 6 , 7 and 8 of form 8606 could not and would not include the value of any contributions whether deductible or not that were done after 12/31.
as far as IRS instructions for the form and in the PUB are concerned they actually have no authoritative value. I believe there is actually a SCOTUS case on this where the taxpayers relied on an IRS pub or instruction. The IRS said they made a mistake because what they wrote was contrary to the Code and Regs. . SCOTUS ruled in favor of the IRS.
