Add is_redundant_value_and_holder() and use to avoid forcing __init__ overrides when they are not needed.

include/pybind11/detail/class.h

@@ -180,6 +180,17 @@ extern "C" inline PyObject *pybind11_meta_getattro(PyObject *obj, PyObject *name
     return PyType_Type.tp_getattro(obj, name);
 }
 
+// Band-aid workaround to fix a subtle but serious bug in a minimalistic fashion. See PR #4762.
+inline bool is_redundant_value_and_holder(const std::vector<type_info *> &bases,
+                                          std::size_t ix_base) {
+    for (std::size_t i = 0; i < ix_base; i++) {
+        if (PyType_IsSubtype(bases[i]->type, bases[ix_base]->type) != 0) {
+            return true;
+        }
+    }
+    return false;
+}
+
 /// metaclass `__call__` function that is used to create all pybind11 objects.
 extern "C" inline PyObject *pybind11_meta_call(PyObject *type, PyObject *args, PyObject *kwargs) {
 
@@ -189,18 +200,20 @@ extern "C" inline PyObject *pybind11_meta_call(PyObject *type, PyObject *args, P
         return nullptr;
     }
 
-    // This must be a pybind11 instance
-    auto *instance = reinterpret_cast<detail::instance *>(self);
-
     // Ensure that the base __init__ function(s) were called
-    for (const auto &vh : values_and_holders(instance)) {
-        if (!vh.holder_constructed()) {
+    const auto &bases = all_type_info((PyTypeObject *) type);
+    values_and_holders vhs(reinterpret_cast<detail::instance *>(self));
+    assert(bases.size() == vhs.size());
+    std::size_t ix_base = 0;
+    for (const auto &vh : vhs) {
+        if (!vh.holder_constructed() && !is_redundant_value_and_holder(bases, ix_base)) {
             PyErr_Format(PyExc_TypeError,
                          "%.200s.__init__() must be called when overriding __init__",
                          get_fully_qualified_tp_name(vh.type->type).c_str());
             Py_DECREF(self);
             return nullptr;
         }
+        ix_base++;
     }
 
     return self;

include/pybind11/detail/type_caster_base.h

@@ -102,6 +102,7 @@ public:
 inline std::pair<decltype(internals::registered_types_py)::iterator, bool>
 all_type_info_get_cache(PyTypeObject *type);
 
+// Band-aid workaround to fix a subtle but serious bug in a minimalistic fashion. See PR #4762.
 inline void all_type_info_add_base_most_derived_first(std::vector<type_info *> &bases,
                                                       type_info *addl_base) {
     for (auto it = bases.begin(); it != bases.end(); it++) {

tests/test_python_multiple_inheritance.py

@@ -8,10 +8,8 @@ class PC(m.CppBase):
     pass
 
 
-class PPCCInit(PC, m.CppDrvd):
-    def __init__(self, value):
-        PC.__init__(self, value)
-        m.CppDrvd.__init__(self, value + 1)
+class PPCC(PC, m.CppDrvd):
+    pass
 
 
 def test_PC():
@@ -21,14 +19,14 @@ def test_PC():
     assert d.get_base_value() == 13
 
 
-def test_PPCCInit():
-    d = PPCCInit(11)
-    assert d.get_drvd_value() == 36
+def test_PPCC():
+    d = PPCC(11)
+    assert d.get_drvd_value() == 33
     d.reset_drvd_value(55)
     assert d.get_drvd_value() == 55
 
-    assert d.get_base_value() == 12
-    assert d.get_base_value_from_drvd() == 12
+    assert d.get_base_value() == 11
+    assert d.get_base_value_from_drvd() == 11
     d.reset_base_value(20)
     assert d.get_base_value() == 20
     assert d.get_base_value_from_drvd() == 20