I asked guidance after the last meeting. I did not get a reply back then
and concluded it as confirming the idea to NOT have the macro.
If someone thinks it is needed after all, please say so.

http://open-std.org/JTC1/SC22/WG21/docs/papers/2020/p1847r3.pdf

On 2/16/2020 4:56 PM, Balog Pal via SG10 wrote:
>> Can you email the sg10 list with an example of why a feature test
> macro may be necessary?
>
> I have one use case in mind. In my projects I usually have a
> configuration/environments checking headers, that looks for compiler,
> language version and platform attributes, with #error if anything is
> not as expected. Especially for things that were assumed and built on.
>
> The paper imposes a strict and reliable order of members, that is not
> unreasonable to assume. And that is easier to check with a related
> than a more generic one.
>
> OTOH my argument in the was that no one is actually swapping.
>
> I'm not in live with the macro and happy to remove it, please make a
> verdict before the postmailing deadline.
>