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 (cherry picked from commit 376ffc6) Co-authored-by: Iurii Kemaev <6885137+hbq1@users.noreply.github.com>

Author: miss-islington

Lib/dataclasses.py

@@ -834,7 +834,7 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen):
  # Only process classes that have been processed by our
  # decorator. That is, they have a _FIELDS attribute.
  base_fields = getattr(b, _FIELDS, None)
- if base_fields:
+ if base_fields is not None:
  has_dataclass_bases = True
  for f in base_fields.values():
  fields[f.name] = f

Lib/test/test_dataclasses.py

@@ -2568,6 +2568,30 @@ class D(C):
  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):

Misc/NEWS.d/next/Library/2021-02-09-07-24-29.bpo-43176.bocNQn.rst

@@ -0,0 +1 @@
+Fixed processing of empty dataclasses.