Add `all_type_info_check_for_divergence()` and some tests.

include/pybind11/detail/type_caster_base.h

@@ -115,6 +115,40 @@ inline void all_type_info_add_base_most_derived_first(std::vector<type_info *> &
     bases.push_back(addl_base);
 }
 
+inline void all_type_info_check_for_divergence(const std::vector<type_info *> &bases) {
+    using sz_t = std::size_t;
+    sz_t n = bases.size();
+    if (n < 3) {
+        return;
+    }
+    std::vector<sz_t> cluster_ids;
+    cluster_ids.reserve(n);
+    for (sz_t ci = 0; ci < n; ci++) {
+        cluster_ids.push_back(ci);
+    }
+    for (sz_t i = 0; i < n - 1; i++) {
+        if (cluster_ids[i] != i) {
+            continue;
+        }
+        for (sz_t j = i + 1; j < n; j++) {
+            if (PyType_IsSubtype(bases[i]->type, bases[j]->type) != 0) {
+                sz_t k = cluster_ids[j];
+                if (k == j) {
+                    cluster_ids[j] = i;
+                } else {
+                    PyErr_Format(
+                        PyExc_TypeError,
+                        "bases include diverging derived types: base=%s, derived1=%s, derived2=%s",
+                        bases[j]->type->tp_name,
+                        bases[k]->type->tp_name,
+                        bases[i]->type->tp_name);
+                    throw error_already_set();
+                }
+            }
+        }
+    }
+}
+
 // Populates a just-created cache entry.
 PYBIND11_NOINLINE void all_type_info_populate(PyTypeObject *t, std::vector<type_info *> &bases) {
     assert(bases.empty());
@@ -168,6 +202,7 @@ PYBIND11_NOINLINE void all_type_info_populate(PyTypeObject *t, std::vector<type_
             }
         }
     }
+    all_type_info_check_for_divergence(bases);
 }
 
 /**

tests/test_python_multiple_inheritance.cpp

@@ -26,6 +26,18 @@ private:
     int drvd_value;
 };
 
+struct CppDrvd2 : CppBase {
+    explicit CppDrvd2(int value) : CppBase(value), drvd2_value(value * 5) {}
+    int get_drvd2_value() const { return drvd2_value; }
+    void reset_drvd2_value(int new_value) { drvd2_value = new_value; }
+
+    int get_base_value_from_drvd2() const { return get_base_value(); }
+    void reset_base_value_from_drvd2(int new_value) { reset_base_value(new_value); }
+
+private:
+    int drvd2_value;
+};
+
 } // namespace test_python_multiple_inheritance
 
 TEST_SUBMODULE(python_multiple_inheritance, m) {
@@ -42,4 +54,11 @@ TEST_SUBMODULE(python_multiple_inheritance, m) {
         .def("reset_drvd_value", &CppDrvd::reset_drvd_value)
         .def("get_base_value_from_drvd", &CppDrvd::get_base_value_from_drvd)
         .def("reset_base_value_from_drvd", &CppDrvd::reset_base_value_from_drvd);
+
+    py::class_<CppDrvd2, CppBase>(m, "CppDrvd2")
+        .def(py::init<int>())
+        .def("get_drvd2_value", &CppDrvd2::get_drvd2_value)
+        .def("reset_drvd2_value", &CppDrvd2::reset_drvd2_value)
+        .def("get_base_value_from_drvd2", &CppDrvd2::get_base_value_from_drvd2)
+        .def("reset_base_value_from_drvd2", &CppDrvd2::reset_base_value_from_drvd2);
 }

tests/test_python_multiple_inheritance.py

@@ -1,6 +1,8 @@
 # Adapted from:
 # https://github.com/google/clif/blob/5718e4d0807fd3b6a8187dde140069120b81ecef/clif/testing/python/python_multiple_inheritance_test.py
 
+import pytest
+
 from pybind11_tests import python_multiple_inheritance as m
 
 
@@ -12,6 +14,22 @@ class PPCC(PC, m.CppDrvd):
     pass
 
 
+class PPPCCC(PPCC, m.CppDrvd2):
+    pass
+
+
+class PC1(m.CppDrvd):
+    pass
+
+
+class PC2(m.CppDrvd2):
+    pass
+
+
+class PCD(PC1, PC2):
+    pass
+
+
 def test_PC():
     d = PC(11)
     assert d.get_base_value() == 11
@@ -33,3 +51,21 @@ def test_PPCC():
     d.reset_base_value_from_drvd(30)
     assert d.get_base_value() == 30
     assert d.get_base_value_from_drvd() == 30
+
+
+def NOtest_PPPCCC():
+    # terminate called after throwing an instance of 'pybind11::error_already_set'
+    # what():  TypeError: bases include diverging derived types:
+    #     base=pybind11_tests.python_multiple_inheritance.CppBase,
+    #     derived1=pybind11_tests.python_multiple_inheritance.CppDrvd,
+    #     derived2=pybind11_tests.python_multiple_inheritance.CppDrvd2
+    PPPCCC(11)
+
+
+def test_PCD():
+    # This escapes all_type_info_check_for_divergence() because CppBase does not appear in bases.
+    with pytest.raises(
+        TypeError,
+        match=r"CppDrvd2\.__init__\(\) must be called when overriding __init__$",
+    ):
+        PCD(11)