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In case it is helpful, here's my list of examples where the AC and existing signature objects are insufficiently expressive:
type(obj)
type(name, bases, mapping)
 two different signatures depending on type 
range(stop)
range(start, stop)
range(start, stop, step)
dict.pop(key[, default])
 default of None has different meaning than missing default
 which raises KeyError when the key is missing
itertools.permutations(iterable[, r])
 where the absence of *r* implies r=len(iterable)
bisect.bisect_right(a, x[, lo[, hi]]) -> index
 where the absence of *hi* implies hi=len(a)
min(iterable, *[, default=obj, key=func]) -> value
min(arg1, arg2, *args, *[, key=func]) -> value
 has two signatures depending on the number of
 positional arguments and a keyword argument
 only used in the first signature. It's implementation
 is also shared with max().
dict() -> new empty dictionary
dict(mapping) -> new dictionary initialized from a mapping object's
 (key, value) pairs
dict(iterable) -> new dictionary initialized as if via:
 d = {}
 for k, v in iterable:
 d[k] = v
dict(**kwargs) -> new dictionary initialized with the name=value pairs
 in the keyword argument list. For example: dict(one=1, two=2)
def sumseq(seq, a=0, b=None):
 # Pure python code with nullable int
 if b is None:
 b = len(seq)
 return sum(seq[a:b])