From b0b20c217331e299c20ab743fbb3dfa4c5eef257 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alex=20Gr=C3=B6nholm?= <alex.gronholm@nextday.fi>
Date: Sun, 3 Nov 2024 12:47:36 +0200
Subject: [PATCH] Fixed explicit checks of PEP 604 unions against
 `types.UnionType`

Fixes #467.
---
 docs/versionhistory.rst    |  2 ++
 src/typeguard/_checkers.py | 14 ++++----------
 tests/test_checkers.py     | 12 ++++++++++++
 3 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/docs/versionhistory.rst b/docs/versionhistory.rst
index bc663f7..3196676 100644
--- a/docs/versionhistory.rst
+++ b/docs/versionhistory.rst
@@ -17,6 +17,8 @@ This library adheres to
 - Fixed checking of protocols on the class level (against ``type[SomeProtocol]``)
   (`#498 <https://github.com/agronholm/typeguard/pull/498>`_)
 - Fixed ``Self`` checks in instance/class methods that have positional-only arguments
+- Fixed explicit checks of PEP 604 unions against ``types.UnionType``
+  (`#467 <https://github.com/agronholm/typeguard/pull/467>`_)
 
 **4.4.0** (2024-10-27)
 
diff --git a/src/typeguard/_checkers.py b/src/typeguard/_checkers.py
index 5856cf9..40ff2d4 100644
--- a/src/typeguard/_checkers.py
+++ b/src/typeguard/_checkers.py
@@ -422,6 +422,7 @@ def check_union(
         )
     finally:
         del errors  # avoid creating ref cycle
+
     raise TypeCheckError(f"did not match any element in the union:\n{formatted_errors}")
 
 
@@ -431,6 +432,9 @@ def check_uniontype(
     args: tuple[Any, ...],
     memo: TypeCheckMemo,
 ) -> None:
+    if not args:
+        return check_instance(value, types.UnionType, (), memo)
+
     errors: dict[str, TypeCheckError] = {}
     try:
         for type_ in args:
@@ -876,16 +880,6 @@ def check_paramspec(
     pass  # No-op for now
 
 
-def check_instanceof(
-    value: Any,
-    origin_type: Any,
-    args: tuple[Any, ...],
-    memo: TypeCheckMemo,
-) -> None:
-    if not isinstance(value, origin_type):
-        raise TypeCheckError(f"is not an instance of {qualified_name(origin_type)}")
-
-
 def check_type_internal(
     value: Any,
     annotation: Any,
diff --git a/tests/test_checkers.py b/tests/test_checkers.py
index 8a2ef9f..d03080b 100644
--- a/tests/test_checkers.py
+++ b/tests/test_checkers.py
@@ -1,5 +1,6 @@
 import collections.abc
 import sys
+import types
 from contextlib import nullcontext
 from functools import partial
 from io import BytesIO, StringIO
@@ -882,6 +883,17 @@ def inner3():
         inner3()
         assert not leaked
 
+    @pytest.mark.skipif(sys.version_info < (3, 10), reason="UnionType requires 3.10")
+    def test_raw_uniontype_success(self):
+        check_type(str | int, types.UnionType)
+
+    @pytest.mark.skipif(sys.version_info < (3, 10), reason="UnionType requires 3.10")
+    def test_raw_uniontype_fail(self):
+        with pytest.raises(
+            TypeCheckError, match="class str is not an instance of types.UnionType"
+        ):
+            check_type(str, types.UnionType)
+
 
 class TestTypevar:
     def test_bound(self):