D81823 review

Hi folks,

A university class I’m involved with would like to teach the spaceship operator this week. I’ve had a few spaceship-related patches open for review since 2020-05-31, but it looks like they’re still in need of some in-depth reviewing before being merged.

I’ve broken the minimal necessary change for the class off into D81823, which is to get std::partial_ordering, std::weak_ordering, and std::strong_ordering self-comparable. Would it be possible for this to be reviewed, and if acceptable, merged within the next day or so please?
It would be great to be able to show off this fantastic new feature!