Add tests using warnings.warn() with Warnings

`warnings.warn()` expects that its first argument is a `str` or a
`Warning`, but since 9454fc3
`pytest.warns()` no longer allows `Warning` instances unless the first
argument the `Warning` was initialized with is a `str`. Furthermore, if
the `Warning` was created without arguments then `pytest.warns()` raises
an unexpected `IndexError`. The new tests reveal the problem.

Author: eerovaher

testing/test_recwarn.py

@@ -480,24 +480,34 @@ def test_catch_warning_within_raise(self) -> None:
                 raise ValueError("some exception")
 
 
-def test_raise_type_error_on_non_string_warning() -> None:
-    """Check pytest.warns validates warning messages are strings (#10865)."""
+def test_raise_type_error_on_invalid_warning() -> None:
+    """Check pytest.warns validates warning messages are strings (#10865) or
+    Warning instances (#11959)."""
     with pytest.raises(TypeError, match="Warning message must be str"):
         with pytest.warns(UserWarning):
             warnings.warn(1)  # type: ignore
 
 
-def test_no_raise_type_error_on_string_warning() -> None:
-    """Check pytest.warns validates warning messages are strings (#10865)."""
-    with pytest.warns(UserWarning):
-        warnings.warn("Warning")
+@pytest.mark.parametrize(
+    "message",
+    [
+        pytest.param("Warning", id="str"),
+        pytest.param(UserWarning(), id="UserWarning"),
+        pytest.param(Warning(), id="Warning"),
+    ],
+)
+def test_no_raise_type_error_on_valid_warning(message: str | Warning) -> None:
+    """Check pytest.warns validates warning messages are strings (#10865) or
+    Warning instances (#11959)."""
+    with pytest.warns(Warning):
+        warnings.warn(message)
 
 
 @pytest.mark.skipif(
     hasattr(sys, "pypy_version_info"),
     reason="Not for pypy",
 )
-def test_raise_type_error_on_non_string_warning_cpython() -> None:
+def test_raise_type_error_on_invalid_warning_message_cpython() -> None:
     # Check that we get the same behavior with the stdlib, at least if filtering
     # (see https://github.com/python/cpython/issues/103577 for details)
     with pytest.raises(TypeError):