If you use the implicit optional type then you have no way to express 'not optional'. If you use the explicit optional then you can express optional/not-optiona

If you use the implicit optional type then you have no way to express 'not optional'. If you use the explicit optional then you can express optional/not-optional but you're stuck with a potential 3rd state. One of these problems can be fixed by convention (don't use null, ever) and the other has no fix. (unless you go down the path of annotations)