Check default argument values against choices

In `parse_args`, check also action default values, so that an exception
is raised if an argument's default is not in `choices`.

Add a test case to verify the exception is raised.

Lib/argparse.py

@@ -2128,8 +2128,9 @@ def consume_positionals(start_index):
                         isinstance(action.default, str) and
                         hasattr(namespace, action.dest) and
                         action.default is getattr(namespace, action.dest)):
-                        setattr(namespace, action.dest,
-                                self._get_value(action, action.default))
+                        value = self._get_value(action, action.default)
+                        self._check_value(action, value)
+                        setattr(namespace, action.dest, value)
 
         if required_actions:
             self.error(_('the following arguments are required: %s') %

Lib/test/test_argparse.py

@@ -5044,6 +5044,15 @@ def test_argument_error(self):
         error = argparse.ArgumentError(None, msg)
         self.assertEqual(str(error), msg)
 
+    def test_default_validates_against_choices(self):
+        parser = ErrorRaisingArgumentParser()
+        parser.add_argument('--choice-with-default', choices=['a', 'b'],
+                            default='foo')
+        with self.assertRaises(ArgumentParserError) as cm:
+            parser.parse_args([])
+        msg = str(cm.exception)
+        self.assertRegex(msg, 'invalid choice')
+
 # =======================
 # ArgumentTypeError tests
 # =======================