Here is a type of test case that I think should be considered for addition (to confirm that the code doesn't double-convert strings in at least one case).  Maybe there is already a test case like this:

class MyString(str): pass

def convert(s):
    return MyString("*" + s)

parser = ArgumentParser()
parser.add_argument("--test", dest="test", type=convert, default="foo")
args = parser.parse_args()

print(args.test)