import functools

class Foo:
    def f(self, x: int) -> int:
        return x

class Bar(Foo):
    @functools.lru_cache()
    def f(self, x: int) -> int:
        return x + 1

crashes with

  File "/home/sully/src/mypy/mypyc/irbuild/function.py", line 379, in handle_ext_method
    and not is_same_method_signature(class_ir.method_decls[name].sig,
over.py:8: KeyError: 'f'

Typically we don't put decorated functions in the vtable since we invoke them through the API. We should generate native glue wrappers that delegate to the decorated callable object, like we do when allowing interpreted subclasses.
(If we implement #481 (storing decorated functions in globals for faster lookup), we should just always use native wrappers to save a dict lookup.)