P3922R1 Missing deduction guide from simd::mask to simd::vec

[eisenwave]

Fixes #8474

No NB comment.
No paper tracker issue yet.

[jwakely]

The itemdecl needs to declare k used in the itemdescr.

[jwakely]

I thought "Fixup" commits were for minor editorial corrections to the paper wording, not for cases where the pull request doesn't correctly implement the paper wording.

Anyway, this is correct now.

[eisenwave]

I thought "Fixup" commits were for minor editorial corrections to the paper wording, not for cases where the pull request doesn't correctly implement the paper wording.

I thought so too. Then I had a conversation last motions where it was brought up that we don't want authors to force-push on these motions because it makes reviewing harder or something, so it's better to add a Fixup commit even for things that weren't proper paper applications in the first place.

To be fair, I don't think it would have killed anyone if I force-pushed this one.