I don't think this should be done.  We want the lru_cache to be a pass-through.  Applying defaults or keyword-only/positional-only restrictions is the responsibility of the inner function.  

FWIW, here are the fields that Nick selected to be included in update_wrapper():  ('__module__', '__name__', '__qualname__', '__doc__', '__annotations__').

Those are sufficient to get help() to work which is all we were aiming for:

>>> from functools import *
>>> @lru_cache
def cached_func(b=5):
    pass

>>> help(cached_func)
Help on _lru_cache_wrapper in module __main__:

cached_func(b=5)