[mikepurvis]

I get that under the hood, a library like eigen has loads of tricks and optimizations to make vector and matrix operations maximally efficient on modern CPUs.

But the comment was specifically about the interface to those operations, that it's convenient and non-harmful ("cute") to have an infix operator just work, because no one would expect that multiplying vectors is going to secretly be a function call that could block or throw or have other unknown side effects the way something like using "stream insertion" on a logger object might. Or have an uncertain effect, like defining operator+ for just one of the multiple possible schemes by which two dicts may be combined.