aaron.ballman added a comment.

In D120862#3857372 <https://reviews.llvm.org/D120862#3857372>, @shafik wrote:

> In D120862#3857340 <https://reviews.llvm.org/D120862#3857340>, @pcc wrote:
>
>> Does that DR apply retroactively to C++17? I get the impression that 
>> "Status: C++20" means that the issue was only fixed in C++20, which would 
>> make this well-formed with `-std=c++17`.
>
> This was simply an oversite in the wording of the paper see discussion here 
> <https://twitter.com/zygoloid/status/965065725126651905?s=20&t=VS6_mRhxXKzho6eWswAfmA>
>  and this was never intended to work.
>
> We have some discretion about how far back to apply DRs and I believe as long 
> as applying them back does not cause major disruption we do CC @aaron.ballman

You're correct that "Status: C++20" means that the issue was fixed in C++20. 
And technically speaking, we only need to apply those changes in C++20 and 
later. However, the intent of the DR process is to fix mistakes in older 
language modes. ISO doesn't let us retroactively change a published standard, 
so we keep this side list of "oh you should also fix this stuff" with the 
intent that the fixes apply as far back as they're relevant. We try to follow 
that guidance whenever we can, but if a DR causes code to break and that turns 
out to be disruptive, we'll sometimes decide to not apply it farther back than 
the standard version it was fixed in. I don't think there's a problem applying 
that specific DR as far back as we can.


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https://reviews.llvm.org/D120862

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