bpo-45663: Fix is_dataclass() for dataclasses which are subclasses of types.GenericAlias (GH-29294)

(cherry picked from commit 446be166861b2f08f87f74018113dd98ca5fca02)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

3 files changed, 15 insertions, 1 deletions

diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index fe4094b891..105a95b955 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -1211,7 +1211,7 @@ def _is_dataclass_instance(obj):
 def is_dataclass(obj):
     """Returns True if obj is a dataclass or an instance of a
     dataclass."""
-    cls = obj if isinstance(obj, type) else type(obj)
+    cls = obj if isinstance(obj, type) and not isinstance(obj, GenericAlias) else type(obj)
     return hasattr(cls, _FIELDS)
 
 
diff --git a/Lib/test/test_dataclasses.py b/Lib/test/test_dataclasses.py
index 47075df8d5..ef5009ab11 100644
--- a/Lib/test/test_dataclasses.py
+++ b/Lib/test/test_dataclasses.py
@@ -8,6 +8,7 @@ import abc
 import pickle
 import inspect
 import builtins
+import types
 import unittest
 from unittest.mock import Mock
 from typing import ClassVar, Any, List, Union, Tuple, Dict, Generic, TypeVar, Optional, Protocol
@@ -1354,6 +1355,17 @@ class TestCase(unittest.TestCase):
                 with self.assertRaisesRegex(TypeError, 'should be called on dataclass instances'):
                     replace(obj, x=0)
 
+    def test_is_dataclass_genericalias(self):
+        @dataclass
+        class A(types.GenericAlias):
+            origin: type
+            args: type
+        self.assertTrue(is_dataclass(A))
+        a = A(list, int)
+        self.assertTrue(is_dataclass(type(a)))
+        self.assertTrue(is_dataclass(a))
+
+
     def test_helper_fields_with_class_instance(self):
         # Check that we can call fields() on either a class or instance,
         #  and get back the same thing.
diff --git a/Misc/NEWS.d/next/Library/2021-10-28-23-11-59.bpo-45663.J90N5R.rst b/Misc/NEWS.d/next/Library/2021-10-28-23-11-59.bpo-45663.J90N5R.rst
new file mode 100644
index 0000000000..f246f67cf8
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-10-28-23-11-59.bpo-45663.J90N5R.rst
@@ -0,0 +1,2 @@
+Fix :func:`dataclasses.is_dataclass` for dataclasses which are subclasses of
+:class:`types.GenericAlias`.