Without significant language changes, this is not possible. While your code may be typed as an int, I can simply redefine what int means. I can also modify the code in your method.
I guess it would work with the ongoing jit work, which (as far as I understood..) run the code "as usual", then notice that a specific variable is always a dict (or whatever). Then it patches the code to run the dict-optimized code by default (and fallback to the generic code if, somehow, the variable is no longer a dict).
With typing, the generic code could be avoided altogether. The algorithm would be:
- notice that some variable can be processed by a dict-optimized code (because its typing is a dict, or something that looks like a dict etc)
- when processing, check that the variable is indeed a "dict", raise an exception if not
- run the optimized code
- if the typing information changes (because the class has been redefined and the variable is no longer a "dict"), then go to step 1 and either stick with the current optimized code, use another one, or use the generic one
- enforce types (you said that variable is a Thing but a Thing was not given: exception)
- improve the jit by removing the bootstrap phase (where the jit watches and then try to guess what could be improved)