I was expecting a horrible mess when I clicked through to TFA, after having read comments like yours, but then... I actually enjoyed what I found there. Maybe it's because I'm quite interested in C++'s ongoing modernization story, even though it's unlikely to ever catch up to Rust (and others) in terms of safety?

Right.

This is the sort of thing that’s interesting to know from an intellectual perspective to see how far can you bend and twist a language to do certain things, but you’re setting yourself up for a world of maintenance pain if you use this sort of stuff in your code base.