Merge cf38460 into bab0b3f

src/typeguard/_checkers.py

@@ -405,6 +405,7 @@ def check_union(
     formatted_errors = indent(
         "\n".join(f"{key}: {error}" for key, error in errors.items()), "  "
     )
+    del errors  # avoid creating ref cycle
     raise TypeCheckError(f"did not match any element in the union:\n{formatted_errors}")
 
 

tests/test_checkers.py

@@ -768,6 +768,26 @@ def test_union_fail(self, annotation, value):
             f"  int: is not an instance of int"
         )
 
+    @pytest.mark.skipif(
+        sys.implementation.name != "cpython",
+        reason="Test relies on CPython's reference counting behavior",
+    )
+    def test_union_reference_leak(self):
+        leaked = True
+
+        class Leak:
+            def __del__(self):
+                nonlocal leaked
+                leaked = False
+
+        def inner():
+            leak = Leak()  # noqa: F841
+            with pytest.raises(TypeCheckError, match="any element in the union:"):
+                check_type(1, Union[str, bytes])
+
+        inner()
+        assert not leaked
+
 
 class TestTypevar:
     def test_bound(self):