Well my point is more that if they introduced an entirely new non-null reference type to the language then you could never make that mistake - it would be non-o

Well my point is more that if they introduced an entirely new non-null reference type to the language then you could never make that mistake - it would be non-optional for those calling into your code to respect whether null can be passed in or not based on the type you used. Since they instead made it an optional system you can not rely on those using a library you make to actually turn it on, you have to accept that null _can_ be passed in even if you mark something as non-null according to the system on your side, which is dumb.