We will probably be pushing people very heavily towards always using specialized arrays for working with nullable objects. I'm a little surprised by your comme

We will probably be pushing people very heavily towards always using specialized arrays for working with nullable objects. I'm a little surprised by your comment about sufficiently smart compilers, since Julia seems to have taken the exact opposite approach to compiler design. The language is systematically designed to require little intelligence from the compiler. Being able to safely drop tag checks inside of a function's body seems like a fairly simple part of the language's design, since the type of all inputs to a function and all function-local variables are, in general, easily knowable as invariants before a function call even begins evaluating.