gh-116647: Fix recursive child in dataclasses (#116790)

Lib/dataclasses.py

@@ -1075,7 +1075,9 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
         cmp_fields = (field for field in field_list if field.compare)
         terms = [f'self.{field.name}==other.{field.name}' for field in cmp_fields]
         field_comparisons = ' and '.join(terms) or 'True'
-        body =  [f'if other.__class__ is self.__class__:',
+        body =  [f'if self is other:',
+                 f' return True',
+                 f'if other.__class__ is self.__class__:',
                  f' return {field_comparisons}',
                  f'return NotImplemented']
         func = _create_fn('__eq__',

Lib/test/test_dataclasses/__init__.py

@@ -2471,6 +2471,15 @@ class TestRepr(unittest.TestCase):
 
 
 class TestEq(unittest.TestCase):
+    def test_recursive_eq(self):
+        # Test a class with recursive child
+        @dataclass
+        class C:
+            recursive: object = ...
+        c = C()
+        c.recursive = c
+        self.assertEqual(c, c)
+
     def test_no_eq(self):
         # Test a class with no __eq__ and eq=False.
         @dataclass(eq=False)

Misc/NEWS.d/next/Library/2024-03-14-09-38-51.gh-issue-116647.h0d_zj.rst

@@ -0,0 +1 @@
+Fix recursive child in dataclasses