argparse: Add test and fix support for pos arg with dest and no name.

argparse/argparse.py

@@ -98,6 +98,8 @@ class ArgumentParser:
             dest = kwargs.get("dest")
             if dest is None:
                 dest = args[0]
+            if not args:
+                args = [dest]
         list.append(
             _Arg(args, dest, action, kwargs.get("nargs", None),
                 const, default, kwargs.get("help", "")))

argparse/test_argparse.py

@@ -3,7 +3,7 @@ import argparse
 parser = argparse.ArgumentParser(description="command line program")
 parser.add_argument("a")
 parser.add_argument("b")
-parser.add_argument("c")
+parser.add_argument(dest="c")
 args = parser.parse_args(["1", "2", "3"])
 assert args.a == "1" and args.b == "2" and args.c == "3"