Fixed explicit checks of PEP 604 unions against types.UnionType

Fixes #467.

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)
 

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,

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):