We had two changes to [[nodiscard]] in Cologne:

- P1301R4: [[nodiscard("should have a reason")]], for C++20
- P1771R1: [[nodiscard]] for constructors, as a DR

Currently, I put both under the 201907 block (
https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations#nodiscard).
But that means we have a [retroactive] C++17 feature whose macro has a
value two years later. Is that fine? Alternatively, we could invent an
earlier value for P1771R1 that's in the C++17 timeline.

Thoughts?

Barry