Set error if it's not set in error_already_set()

include/pybind11/cast.h

@@ -106,10 +106,15 @@ PYBIND11_NOINLINE inline handle get_type_handle(const std::type_info &tp) {
 }
 
 PYBIND11_NOINLINE inline std::string error_string() {
-   PyObject *type, *value, *traceback;
+    if (!PyErr_Occurred()) {
-   PyErr_Fetch(&type, &value, &traceback);
+        PyErr_SetString(PyExc_RuntimeError, "Unknown internal error occurred");
+        return "Unknown internal error occurred";
+    }
 
-   std::string errorString;
+    PyObject *type, *value, *traceback;
+    PyErr_Fetch(&type, &value, &traceback);
+
+    std::string errorString;
     if (type) {
         errorString += handle(type).attr("__name__").cast<std::string>();
         errorString += ": ";

tests/test_exceptions.cpp

@@ -104,5 +104,23 @@ test_initializer custom_exceptions([](py::module &m) {
     m.def("throws3", &throws3);
     m.def("throws4", &throws4);
     m.def("throws_logic_error", &throws_logic_error);
-});
 
+    m.def("throw_already_set", [](bool err) {
+        if (err)
+            PyErr_SetString(PyExc_ValueError, "foo");
+        try {
+            throw py::error_already_set();
+        } catch (const std::runtime_error& e) {
+            if ((err && e.what() != std::string("ValueError: foo")) ||
+                (!err && e.what() != std::string("Unknown internal error occurred")))
+            {
+                PyErr_Clear();
+                throw std::runtime_error("error message mismatch");
+            }
+        }
+        PyErr_Clear();
+        if (err)
+            PyErr_SetString(PyExc_ValueError, "foo");
+        throw py::error_already_set();
+    });
+});

tests/test_exceptions.py

@@ -1,6 +1,18 @@
 import pytest
 
 
+def test_error_already_set(msg):
+    from pybind11_tests import throw_already_set
+
+    with pytest.raises(RuntimeError) as excinfo:
+        throw_already_set(False)
+    assert msg(excinfo.value) == "Unknown internal error occurred"
+
+    with pytest.raises(ValueError) as excinfo:
+        throw_already_set(True)
+    assert msg(excinfo.value) == "foo"
+
+
 def test_custom(msg):
     from pybind11_tests import (MyException, throws1, throws2, throws3, throws4,
                                 throws_logic_error)