The situation for type=int and unspecified nargs or nargs="?" is also surprising: 

Python 3.8.3 (default, May 21 2020, 12:19:36) 
[GCC 9.2.1 20191008] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import argparse
>>> p = argparse.ArgumentParser()
>>> p.add_argument("--c", action="extend", type=int)
_ExtendAction(option_strings=['--c'], dest='c', nargs=None, const=None, default=None, type=<class 'int'>, choices=None, help=None, metavar=None)
>>> p.parse_args("--c 1".split())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 1768, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 1800, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 2006, in _parse_known_args
    start_index = consume_optional(start_index)
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 1946, in consume_optional
    take_action(action, args, option_string)
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 1874, in take_action
    action(self, namespace, argument_values, option_string)
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 1171, in __call__
    items.extend(values)
TypeError: 'int' object is not iterable
>>> p = argparse.ArgumentParser()
>>> p.add_argument("--c", action="extend", type=int, nargs="?")
_ExtendAction(option_strings=['--c'], dest='c', nargs='?', const=None, default=None, type=<class 'int'>, choices=None, help=None, metavar=None)
>>> p.parse_args("--c 1".split())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 1768, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 1800, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 2006, in _parse_known_args
    start_index = consume_optional(start_index)
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 1946, in consume_optional
    take_action(action, args, option_string)
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 1874, in take_action
    action(self, namespace, argument_values, option_string)
  File "/home/jonathan/.pyenv/versions/3.8.3/lib/python3.8/argparse.py", line 1171, in __call__
    items.extend(values)
TypeError: 'int' object is not iterable
>>>

I suggest that the default nargs for extend should be "*" or "+" and an exception should be raised if nargs is given as "?". I don't see the current behaviour with unspecified nargs or nargs="?" being useful (and it certainly is surprising). In both cases, I think the least surprising behaviour would be for extend to act the same as append (or for an exception to be raised).

> But I wonder, was this situation discussed in the original bug/issue?
Doesn't look like it:
https://bugs.python.org/issue23378
https://github.com/python/cpython/commit/aa32a7e1116f7aaaef9fec453db910e90ab7b101