[3.10] bpo-44676: Add ability to serialize types.Union (GH-27244) (GH-27333)

(cherry picked from commit fe13f0b0f696464dd6f283576668dbf57cb11399)

Co-authored-by: Yurii Karabas <1998uriyyo@gmail.com>

2 files changed, 37 insertions, 2 deletions

diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index 99b011e..d149e6b 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -13,6 +13,9 @@ import unittest.mock
 import weakref
 import typing
 
+
+T = typing.TypeVar("T")
+
 class Example:
     pass
 
@@ -802,6 +805,38 @@ class UnionTests(unittest.TestCase):
         eq(x[NT], int | NT | bytes)
         eq(x[S], int | S | bytes)
 
+    def test_union_pickle(self):
+        alias = list[T] | int
+        s = pickle.dumps(alias)
+        loaded = pickle.loads(s)
+        self.assertEqual(alias, loaded)
+        self.assertEqual(alias.__args__, loaded.__args__)
+        self.assertEqual(alias.__parameters__, loaded.__parameters__)
+
+    def test_union_from_args(self):
+        with self.assertRaisesRegex(
+                TypeError,
+                r"^Each union argument must be a type, got 1$",
+        ):
+            types.Union._from_args((1,))
+
+        with self.assertRaisesRegex(
+                TypeError,
+                r"Union._from_args\(\) argument 'args' must be tuple, not int$",
+        ):
+            types.Union._from_args(1)
+
+        with self.assertRaisesRegex(ValueError, r"args must be not empty"):
+            types.Union._from_args(())
+
+        alias = types.Union._from_args((int, str, T))
+
+        self.assertEqual(alias.__args__, (int, str, T))
+        self.assertEqual(alias.__parameters__, (T,))
+
+        result = types.Union._from_args((int,))
+        self.assertIs(int, result)
+
     def test_union_parameter_substitution_errors(self):
         T = typing.TypeVar("T")
         x = int | T
diff --git a/Lib/typing.py b/Lib/typing.py
index ffd35e5..e2fb7d1 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -321,7 +321,7 @@ def _eval_type(t, globalns, localns, recursive_guard=frozenset()):
         if isinstance(t, GenericAlias):
             return GenericAlias(t.__origin__, ev_args)
         if isinstance(t, types.Union):
-            return functools.reduce(operator.or_, ev_args)
+            return types.Union._from_args(ev_args)
         else:
             return t.copy_with(ev_args)
     return t
@@ -1800,7 +1800,7 @@ def _strip_annotations(t):
         stripped_args = tuple(_strip_annotations(a) for a in t.__args__)
         if stripped_args == t.__args__:
             return t
-        return functools.reduce(operator.or_, stripped_args)
+        return types.Union._from_args(stripped_args)
 
     return t