Gathers information about the call signature of `func`.

def get_signature(func):
    """
    Gathers information about the call signature of `func`.
    """
    code = func.__code__

    # Names of regular parameters
    parameters = tuple(code.co_varnames[:code.co_argcount])

    # Flags
    has_varargs = bool(code.co_flags & inspect.CO_VARARGS)
    has_varkw = bool(code.co_flags & inspect.CO_VARKEYWORDS)
    has_kwonly = bool(code.co_kwonlyargcount)

    # A mapping of parameter names to default values
    default_values = func.__defaults__ or ()
    defaults = dict(zip(parameters[-len(default_values):], default_values))

    # Type annotations for all parameters
    type_hints = typing.get_type_hints(func) if typing else func.__annotations__
    types = tuple(normalize_type(type_hints.get(param, AnyType)) for param in parameters)

    # Type annotations for required parameters
    required = types[:-len(defaults)] if defaults else types

    # Complexity
    complexity = tuple(map(type_complexity, types)) if typing else None

    return Signature(parameters, types, complexity, defaults, required,
                     has_varargs, has_varkw, has_kwonly)