[Backport maintenance/4.0.x] Fix base class inference for dataclasses with PEP 695 syntax (#2919)

pylint-backport[bot] · cdce8p · web-flow · commit ae761dca40d1 · 2025-12-26T13:18:27.000+01:00
(cherry picked from commit 3774c0a) Co-authored-by: Marc Mueller <30130371+cdce8p@users.noreply.github.com>
diff --git a/ChangeLog b/ChangeLog
@@ -13,6 +13,10 @@ What's New in astroid 4.0.3?
 ============================
 Release date: TBA
 
+* Fix base class inference for dataclasses using the PEP 695 typing syntax.
+
+  Refs pylint-dev/pylint#10788
+
 
 
 What's New in astroid 4.0.2?
diff --git a/astroid/brain/brain_dataclasses.py b/astroid/brain/brain_dataclasses.py
@@ -54,7 +54,7 @@ def is_decorated_with_dataclass(
     )
 
 
-def dataclass_transform(node: nodes.ClassDef) -> None:
+def dataclass_transform(node: nodes.ClassDef) -> nodes.ClassDef | None:
     """Rewrite a dataclass to be easily understood by pylint."""
     node.is_dataclass = True
 
@@ -70,7 +70,7 @@ def dataclass_transform(node: nodes.ClassDef) -> None:
         node.instance_attrs[name] = [rhs_node]
 
     if not _check_generate_dataclass_init(node):
-        return
+        return None
 
     kw_only_decorated = False
     if node.decorators.nodes:
@@ -102,6 +102,7 @@ def dataclass_transform(node: nodes.ClassDef) -> None:
             new_assign = parse(f"{DEFAULT_FACTORY} = object()").body[0]
             new_assign.parent = root
             root.locals[DEFAULT_FACTORY] = [new_assign.targets[0]]
+    return node
 
 
 def _get_dataclass_attributes(
diff --git a/tests/test_scoped_nodes.py b/tests/test_scoped_nodes.py
@@ -29,7 +29,7 @@
     util,
 )
 from astroid.bases import BoundMethod, Generator, Instance, UnboundMethod
-from astroid.const import WIN32
+from astroid.const import PY312_PLUS, WIN32
 from astroid.exceptions import (
     AstroidBuildingError,
     AttributeInferenceError,
@@ -1967,6 +1967,34 @@ class E(C[str], D): ...
             cls, [".E", ".C", ".A", ".B", "typing.Generic", ".D", "builtins.object"]
         )
 
+    @pytest.mark.skipif(not PY312_PLUS, reason="PEP 695 syntax requires Python 3.12")
+    def test_mro_generic_8(self):
+        cls = builder.extract_node(
+            """
+        class A: ...
+        class B[T]: ...
+        class C[T](A, B[T]): ...
+        """
+        )
+        assert isinstance(cls, nodes.ClassDef)
+        self.assertEqualMroQName(cls, [".C", ".A", ".B", "builtins.object"])
+
+    @pytest.mark.skipif(not PY312_PLUS, reason="PEP 695 syntax requires Python 3.12")
+    def test_mro_generic_9(self):
+        cls = builder.extract_node(
+            """
+        from dataclasses import dataclass
+        @dataclass
+        class A: ...
+        @dataclass
+        class B[T]: ...
+        @dataclass
+        class C[T](A, B[T]): ...
+        """
+        )
+        assert isinstance(cls, nodes.ClassDef)
+        self.assertEqualMroQName(cls, [".C", ".A", ".B", "builtins.object"])
+
     def test_mro_generic_error_1(self):
         cls = builder.extract_node(
             """
