> What about the things you pass into the function? You can't know their properties in advance, in general, or can you?

You can know that, e.g. a function passed in as an argument can be dereferenced as a pointer stored in a specific offset on the stack.

If your typechecks are compiler-generated statements inserted before a function call, then if you know at compile-time (via declarations or type inference, or type propagation) that a certain variable (to be passed as an argument) will always be of the correct type T, then you can omit the typecheck, as well as boxing/tagging.

You can get really close to the metal this way, and still have your language be as dynamic as can be.