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Here's a possible solution to the problem (assuming there really is one):
- redefine REMAINDER so it matches a '((?:A[AO]*)?)' pattern (where O is a string that looks like an optional flag, A an argument string). I've added the condition that the first match (if any) must be an A. It ends up being closer to the pattern for PARSER.
I included a patch from issue 15112, which delays the consumption of a positional that matches with 0 strings.
In the sample case for this issue, results with this patch are:
 args = parser.parse_args(['app', '--config', 'bar'])
 # Namespace(app='app', app_args=[], config='bar')
 args = parser.parse_args(['--config', 'bar', 'app'])
 # Namespace(app='app', app_args=[], config='bar')
 args = parser.parse_args(['app', 'args', '--config', 'bar'])
 # Namespace(app='app', app_args=['args', '--config', 'bar'], config=None)
In the last case, 'app_args' gets the rest of the strings because the first is a plain 'args'. I believe this is consistent with the intuition expressed in this issue.
I've added one test case to test_argparse.TestNargsRemainder. This is a TestCase that is similar to the above example.
 argument_signatures = [Sig('x'), Sig('y', nargs='...'), Sig('-z')]
 failures = ['', '-z', '-z Z']
 successes = [
 ('X', NS(x='X', y=[], z=None)),
 ('-z Z X', NS(x='X', y=[], z='Z')),
 ('X A B -z Z', NS(x='X', y=['A', 'B', '-z', 'Z'], z=None)),
 ('X Y --foo', NS(x='X', y=['Y', '--foo'], z=None)),
 ('X -z Z A B', NS(x='X', y=['A', 'B'], z='Z')), # new case
 ]
This patch runs test_argparse fine. But there is a slight possibility that this patch will cause backward compatibility problems. Some user might expect y=['-z','Z',...]. But that expectation has not been enshrined the test_argparse.
It may require a slight change to the documentation as well.