Add error_already_set_what what tests, asserting the status quo.

Author: rwgk

tests/test_exceptions.cpp

@@ -299,4 +299,12 @@ TEST_SUBMODULE(exceptions, m) {
             std::throw_with_nested(std::runtime_error("Outer Exception"));
         }
     });
+
+    m.def("error_already_set_what", [](const py::object &exc_type, const py::object &exc_value) {
+        PyErr_SetObject(exc_type.ptr(), exc_value.ptr());
+        std::string what = py::error_already_set().what();
+        bool py_err_set_after_what = (PyErr_Occurred() != nullptr);
+        PyErr_Clear();
+        return py::make_tuple(what, py_err_set_after_what);
+    });
 }

tests/test_exceptions.py

@@ -270,3 +270,54 @@ def test_local_translator(msg):
         m.throws_local_simple_error()
     assert not isinstance(excinfo.value, cm.LocalSimpleException)
     assert msg(excinfo.value) == "this mod"
+
+
+class FlakyException(Exception):
+    def __init__(self, failure_point):
+        if failure_point == "failure_point_init":
+            raise ValueError("triggered_failure_point_init")
+        self.failure_point = failure_point
+
+    def __str__(self):
+        if self.failure_point == "failure_point_str":
+            raise ValueError("triggered_failure_point_str")
+        return "FlakyException.__str__"
+
+
+@pytest.mark.parametrize(
+    "exc_type, exc_value, expected_what",
+    (
+        (ValueError, "plain_str", "ValueError: plain_str"),
+        (ValueError, ("tuple_elem",), "ValueError: ('tuple_elem',)"),
+        (FlakyException, ("happy",), "FlakyException: ('happy',)"),
+    ),
+)
+def test_error_already_set_what_with_happy_exceptions(
+    exc_type, exc_value, expected_what
+):
+    what, py_err_set_after_what = m.error_already_set_what(exc_type, exc_value)
+    assert not py_err_set_after_what
+    assert what == expected_what
+
+
+def test_flaky_exception_failure_point_init():
+    what, py_err_set_after_what = m.error_already_set_what(
+        FlakyException, ("failure_point_init",)
+    )
+    assert not py_err_set_after_what
+    lines = what.splitlines()
+    # PyErr_NormalizeException replaces the original FlakyException with ValueError:
+    assert lines[:3] == ["FlakyException: ('failure_point_init',)", "", "At:"]
+    # Checking the first two lines of the traceback as formatted in error_string(),
+    # which is actually for a different exception (ValueError)!
+    assert "test_exceptions.py(" in lines[3]
+    assert lines[3].endswith("): __init__")
+    assert lines[4].endswith("): test_flaky_exception_failure_point_init")
+
+
+def test_flaky_exception_failure_point_str():
+    what, py_err_set_after_what = m.error_already_set_what(
+        FlakyException, ("failure_point_str",)
+    )
+    assert not py_err_set_after_what
+    assert what == "FlakyException: ('failure_point_str',)"