> someone out there has made assumptions which will now be invalid I was thinking about this as well, but I think the key is that they guarantee only the seman

> someone out there has made assumptions which will now be invalid I was thinking about this as well, but I think the key is that they guarantee only the semantics that are _documented_. If you make assumptions beyond the documentation, then no, of course they can't guarantee compatibility, otherwise it would essentially forbid any changes whatsoever. Still, it is a good thing the Go team takes the extra step to ensure that even changes that fall inside their compatibility promise are _still_ vetted against a vast body of real-world code, and to check that common (non-documented) assumptions won't be violated if they can help it.