I don't think this is a bug because if there are no positional arguments, we can assume that the positional argument "foo" has a null value at the end of the command, which will then overwrite the value of "--foo".

Modifying your example to more clearly show the defaults:

parser = argparse.ArgumentParser()
parser.add_argument('foo', nargs='*', default='pos')
parser.add_argument('--foo', default='opt');

The positional's default has priority:

In [22]: parser.parse_args([]) 
Out[22]: Namespace(foo='pos')

With a positional value:

In [25]: parser.parse_args(['bar'])
Out[25]: Namespace(foo=['bar'])

With both, the last optional's value is kept:

In [26]: parser.parse_args(['bar','--foo=x'])
Out[26]: Namespace(foo='x')

But here, the positional default is used:

In [27]: parser.parse_args(['--foo=x'])
Out[27]: Namespace(foo='pos')

There some subtilties in how positionals and optionals are parsed, and how the '*' positional in particular is handled.

A star positional is 'satisfied' by "nothing", an empty list of strings. That means it is always "seen", in both the [22] and the [27] cases. That empty list will be put in the namespace. Except if it has a not-None default, that is put there instead. That behavior was included in part so that such a positional can be used in a mutually_exclusive_group.

Normally parsing alternates between handling positionals and optionals. That's the key loop in _parse_known_args. But if a positional doesn't consume any strings, it is skipped in this loop. There is a final `consume_positionals' that will cleanup any remaining positionals, including 'star' ones.

In [27], the '--foo' is handled, but the final consumption of the positional ends up over writing that value with its default (or [] in your example).

Hopefully that explains why we get a counter intuitive result. Sharing a dest can be convenient, but it is possible as a consequence of how the overall parsing is done, not as an intentional feature. Nothing in the code takes note that the two Actions share the dest. Each does its own thing, without awareness that it is sharing the dest. The last one 'wins'.

So such sharing must be done with caution. If it causes problems like this, don't do it.