Validate root values in check_value

dargs/dargs.py

@@ -480,6 +480,9 @@ def check_value(
         """
         if allow_ref:
             value = deepcopy(value)
+        # ``traverse_value`` only checks descendants, so validate the root value
+        # explicitly before descending into any sub-fields or variants.
+        self._check_data(value, [])
         self.traverse_value(
             value,
             key_hook=Argument._check_exist,

tests/test_checker.py

@@ -100,6 +100,16 @@ def test_sub_fields(self) -> None:
         with self.assertRaises(ValueError):
             Argument("base", dict, [Argument("sub1", int), Argument("sub1", int)])
 
+    def test_check_value_validates_root(self) -> None:
+        """Root types and extra checks are enforced by check_value()."""
+        with self.assertRaises(ArgumentTypeError):
+            Argument("value", int).check_value("not an integer")
+
+        positive = Argument("value", int, extra_check=lambda value: value > 0)
+        positive.check_value(1)
+        with self.assertRaises(ArgumentValueError):
+            positive.check_value(0)
+
     def test_sub_repeat_list(self) -> None:
         ca = Argument(
             "base", list, [Argument("sub1", int), Argument("sub2", str)], repeat=True