Add additional info to TypeError when C++->Python casting fails (#3605)

* Add additional info to TypeInfo when C++->Python casting fails

* Fix typo

* Address reviewer comments

include/pybind11/pybind11.h

@@ -996,6 +996,13 @@ protected:
                               "Python type! The signature was\n\t";
             msg += it->signature;
             append_note_if_missing_header_is_suspected(msg);
+#if PY_VERSION_HEX >= 0x03030000
+            // Attach additional error info to the exception if supported
+            if (PyErr_Occurred()) {
+                raise_from(PyExc_TypeError, msg.c_str());
+                return nullptr;
+            }
+#endif
             PyErr_SetString(PyExc_TypeError, msg.c_str());
             return nullptr;
         }

tests/test_operator_overloading.cpp

@@ -7,10 +7,11 @@
     BSD-style license that can be found in the LICENSE file.
 */
 
-#include "pybind11_tests.h"
 #include "constructor_stats.h"
-#include <pybind11/operators.h>
+#include "pybind11_tests.h"
 #include <functional>
+#include <pybind11/operators.h>
+#include <pybind11/stl.h>
 
 class Vector2 {
 public:
@@ -71,6 +72,12 @@ int operator+(const C2 &, const C2 &) { return 22; }
 int operator+(const C2 &, const C1 &) { return 21; }
 int operator+(const C1 &, const C2 &) { return 12; }
 
+struct HashMe {
+    std::string member;
+};
+
+bool operator==(const HashMe &lhs, const HashMe &rhs) { return lhs.member == rhs.member; }
+
 // Note: Specializing explicit within `namespace std { ... }` is done due to a
 // bug in GCC<7. If you are supporting compilers later than this, consider
 // specializing `using template<> struct std::hash<...>` in the global
@@ -82,6 +89,14 @@ namespace std {
         // Not a good hash function, but easy to test
         size_t operator()(const Vector2 &) { return 4; }
     };
+
+    // HashMe has a hash function in C++ but no `__hash__` for Python.
+    template <>
+    struct hash<HashMe> {
+        std::size_t operator()(const HashMe &selector) const {
+            return std::hash<std::string>()(selector.member);
+        }
+    };
 } // namespace std
 
 // Not a good abs function, but easy to test.
@@ -228,8 +243,12 @@ TEST_SUBMODULE(operators, m) {
         .def("__hash__", &Hashable::hash)
         .def(py::init<int>())
         .def(py::self == py::self);
-}
 
+    // define __eq__ but not __hash__
+    py::class_<HashMe>(m, "HashMe").def(py::self == py::self);
+
+    m.def("get_unhashable_HashMe_set", []() { return std::unordered_set<HashMe>{{"one"}}; });
+}
 #if !defined(_MSC_VER) && !defined(__INTEL_COMPILER)
   #pragma GCC diagnostic pop
 #endif

tests/test_operator_overloading.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 import pytest
 
+import env
 from pybind11_tests import ConstructorStats
 from pybind11_tests import operators as m
 
@@ -135,8 +136,9 @@ def test_overriding_eq_reset_hash():
     assert m.Comparable(15) is not m.Comparable(15)
     assert m.Comparable(15) == m.Comparable(15)
 
-    with pytest.raises(TypeError):
-        hash(m.Comparable(15))  # TypeError: unhashable type: 'm.Comparable'
+    with pytest.raises(TypeError) as excinfo:
+        hash(m.Comparable(15))
+    assert str(excinfo.value).startswith("unhashable type:")
 
     for hashable in (m.Hashable, m.Hashable2):
         assert hashable(15) is not hashable(15)
@@ -144,3 +146,10 @@ def test_overriding_eq_reset_hash():
 
         assert hash(hashable(15)) == 15
         assert hash(hashable(15)) == hash(hashable(15))
+
+
+def test_return_set_of_unhashable():
+    with pytest.raises(TypeError) as excinfo:
+        m.get_unhashable_HashMe_set()
+    if not env.PY2:
+        assert str(excinfo.value.__cause__).startswith("unhashable type:")