I published my trip report about the Hagenberg meeting last week: https://www.think-cell.com/en/career/devblog/trip-report-winter-iso-cpp-meeting-in-hagenberg-austria
It was pointed out to me that I was wrong about the potential for dangerous optimizations with contracts and ODR. It is in fact not allowed based on the assumption that a check happens if it involves a weak symbol. The blog post has been updated.