Backdoor ira/Mega backdoor ira: conversion/本金能随时取出

这个话题的讨论一直很confusin。我看了一些英文文章，不少都是错的（即和我理解不一样）。不过好在所有的讨论贴(无论中英文）都是对的，包括地理的三个帖子。

本帖个人理解，希望能帮到同样疑惑的人，错了请指正。

先说结论：Backdoor ira或Mega backdoor ira: conversion(国人一般称之为本) 能随时取出，无penalty无税
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两个假设：
1.你的收入已经高到不能直接contribute roth ira （不然你也不用backdoor了）
2.你的收入已经高到你contribute traditional ira已经不抵税了，这种contribution叫non-deductable contribution

一个前提：
操作正确 (具体就是，放进 traditional ira或after tax 401k里的钱没有放太久/没有涨太多）

一个知识点：
如果符合假设1, 那你通过traditional ira或after-tax 401k或roth 401k转进roth ira的钱，不叫本金，而是叫conversion或者roll over。咱国人喜欢把这个叫本金，讨论的时候要注意区分。
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正文：

https://www.irs.gov/publications ... 9_publink1000231064 -> Roth IRA部分-> Additional Tax on Early Distributions部分 -> Distributions of conversion and certain rollover contributions within 5-year period部分:
If, within the 5-year period starting with the first day of your tax year in which you convert an amount from a traditional IRA or roll over an amount from a qualified retirement plan to a Roth IRA, you take a distribution from a Roth IRA, you may have to pay the 10% additional tax on early distributions. You must generally pay the 10% additional tax on any amount attributable to the part of the amount converted or rolled over (the conversion or rollover contribution) that you had to include in income (recapture amount).
翻译： 如果提早提取，convert或roll over的钱中，那些应该include in income的conversion要交10% penalty (这个数额又叫recapture amount)

recapture amount是什么呢：
在https://www.irs.gov/instructions/i5329#idm140008360029632 里的 Recapture amount subject to the additional tax on early distributions部分：
The recapture amount is the sum of the early distribution amounts that you allocate to these taxable portions of your conversions or rollovers.
重点：sum of TAXABLE portions

TAXABLE是啥意思呢：
在https://www.irs.gov/publications/p590b# Distributions Fully or Partly Taxable部分 -> Partly taxable部分（虽然这部分是traditional ira的，但对roth ira也适用，因为这里是介绍Taxable的）：
If you made nondeductible contributions or rolled over any after-tax amounts to any of your traditional IRAs, you have a cost basis (investment in the contract) equal to the amount of those contributions. These nondeductible contributions aren't taxed when they are distributed to you. They are a return of your investment in your IRA.
重点：nondeductible contributions or rolled over any after-tax amounts是不会被taxed的

Only the part of the distribution that represents nondeductible contributions and rolled over after-tax amounts (your cost basis) is tax free. If nondeductible contributions have been made or after-tax amounts have been rolled over to your IRA, distributions consist partly of nondeductible contributions (basis) and partly of deductible contributions, earnings, and gains (if there are any). Until all of your basis has been distributed, each distribution is partly nontaxable and partly taxable.
重点: 只有nondeductible的roll over is tax free. 如果你取出的钱里，有partly deductible contribution (根据上文的假设2，这部分为0)+ earning（根据上文的前提2，这部分约为0），那这一小部分就不是tax free的。 所以不用担心
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其他讨论贴：
https://www.1point3acres.com/bbs/thread-597491-1-1.html  （@诸神黄昏泪 ）
https://www.1point3acres.com/bbs/thread-554054-6-1.html  （@ryd994）
https://www.1point3acres.com/bbs/thread-580659-1-1.html （@gu0001hi ）https://money.stackexchange.com/ ... -a-roth-ira-after-a
https://money.stackexchange.com/ ... -roth-ira-without-p
https://thefinancebuff.com/rollo ... -before-59-1-2.html

价值千金的帖子！