What I'm curious about - this function was already const, so for stuff like this, I'd think there's basically a 100% chance the compiler would optimize this too, just implicitly.

AFAIK this new feature is just for times when it isn't an optimization, but more your own domain invariants. E.g. assertions.

But I could be wrong. I wonder if this can be used for actual optimizations in some places that the compiler couldn't figure out by itself.