Host bindings, DOM in particular, use subclasses a lot. We would need some ability to do type discrimination. For instance, if I were to receive a DOMEvent object, I would likely want to be able to determine that this is DOMMouseEvent, and be able to call DOMMouseEvent-y things on it.

The details on runtime typechecking in the current proposal are vague (it just says "Throw TypeError (as now) if the wrong type is stored in a Table." but does not elaborate on what "the wrong type" means). Assuming that this is a strict 1:1 type-check, we would need some way to do runtime type discrimination, and a way to cast from one type to another.

Possible idea: a CAST instruction which takes two table indexes src and dest. The instruction checks src instanceof TableType(dest). If the check passes, 1 is pushed to the stack and the object's address copied into dest, so it exists in both tables. Otherwise, 0 is pushed to the stack. Cleanup for both src and dest is left to native code.

A variant of the instruction which takes a table index and a typeidx might also be entertained, in case of if (foo instanceof Type) checks that have no need to access the resulting casted object.