bpo-43176: Fix processing of empty dataclasses (GH-24484)

When a dataclass inherits from an empty base, all immutability checks are omitted. This PR fixes this and adds tests for it.

Automerge-Triggered-By: GH:ericvsmith

1 files changed, 24 insertions, 0 deletions

diff --git a/Lib/test/test_dataclasses.py b/Lib/test/test_dataclasses.py
index 5515ca4..12c1918 100644
--- a/Lib/test/test_dataclasses.py
+++ b/Lib/test/test_dataclasses.py
@@ -2594,6 +2594,30 @@ class TestFrozen(unittest.TestCase):
         self.assertEqual(d.i, 0)
         self.assertEqual(d.j, 10)
 
+    def test_inherit_nonfrozen_from_empty_frozen(self):
+        @dataclass(frozen=True)
+        class C:
+            pass
+
+        with self.assertRaisesRegex(TypeError,
+                                    'cannot inherit non-frozen dataclass from a frozen one'):
+            @dataclass
+            class D(C):
+                j: int
+
+    def test_inherit_nonfrozen_from_empty(self):
+        @dataclass
+        class C:
+            pass
+
+        @dataclass
+        class D(C):
+            j: int
+
+        d = D(3)
+        self.assertEqual(d.j, 3)
+        self.assertIsInstance(d, C)
+
     # Test both ways: with an intermediate normal (non-dataclass)
     #  class and without an intermediate class.
     def test_inherit_nonfrozen_from_frozen(self):