> Small vectors break iterator guarantees, for one thing.

It only breaks swap of the container itself during iteration. Which is a super niche condition.

And that swap also invalidates some of std::vector's iterators as well - specifically the end() iterator.

> They also really only make sense for tiny objects (ints, etc.) given you don't want a pickup truck's worth of data on your stack. They're most definitely not general-purpose.

Of course they are still general-purpose. They can (and do) specialize on the size of the object being contained. The only reason std::vector doesn't also have SSO is because it's an ABI break. Not because it's better in some way or less fragile. Legacy is the only reason.

> And that swap also invalidates some of std::vector's iterators as well - specifically the end() iterator.

And they don't invalidate the iterators that point to actual elements, which was kind of the entire point I was making. Don't let that stop you from trying to make it look like I'm just blurting out nonsense, though.

You made a broad, vague claim that iterator guarantees were broken. You misrepresented it as being a much larger issue that it actually is. Nearly all iterator guarantees are not broken. One very specific guarantee in one very specific case is, that's it. And it's a rare, not general, case at that, making the trade-off necessary to achieve it a bad default.

returning, moving, swapping a small vector would break any pointer to an element. That's a big deal. Interior pointers are used all the time (that's, more than performance, the primary reason that reserve exist).

how do small vectors break iterator guarantees?

By invalidating iterators (and pointers and references) when swapping: https://stackoverflow.com/a/8191356

Not sure about iterators, but they do break reference guarantees. Moving a vector doesn't invalidate pointers to the contained elements.