Fix dynamic attribute inheritance in C++

`PyType_Ready` would usually perform the inheritance for us, but it
can't adjust `tp_basicsize` appropriately.

include/pybind11/attr.h

@@ -179,6 +179,9 @@ struct type_record {
 
         bases.append((PyObject *) base_info->type);
 
+        if (base_info->type->tp_dictoffset != 0)
+            dynamic_attr = true;
+
         if (caster)
             base_info->implicit_casts.push_back(std::make_pair(type, caster));
     }

tests/test_methods_and_attributes.cpp

@@ -59,6 +59,8 @@ public:
     ~DynamicClass() { print_destroyed(this); }
 };
 
+class CppDerivedDynamicClass : public DynamicClass { };
+
 test_initializer methods_and_attributes([](py::module &m) {
     py::class_<ExampleMandA>(m, "ExampleMandA")
         .def(py::init<>())
@@ -90,4 +92,7 @@ test_initializer methods_and_attributes([](py::module &m) {
 
     py::class_<DynamicClass>(m, "DynamicClass", py::dynamic_attr())
         .def(py::init());
+
+    py::class_<CppDerivedDynamicClass, DynamicClass>(m, "CppDerivedDynamicClass")
+        .def(py::init());
 });

tests/test_methods_and_attributes.py

@@ -48,7 +48,7 @@ def test_methods_and_attributes():
 
 
 def test_dynamic_attributes():
-    from pybind11_tests import DynamicClass
+    from pybind11_tests import DynamicClass, CppDerivedDynamicClass
 
     instance = DynamicClass()
     assert not hasattr(instance, "foo")
@@ -76,16 +76,17 @@ def test_dynamic_attributes():
     assert cstats.alive() == 0
 
     # Derived classes should work as well
-    class Derived(DynamicClass):
+    class PythonDerivedDynamicClass(DynamicClass):
         pass
 
-    derived = Derived()
-    derived.foobar = 100
-    assert derived.foobar == 100
+    for cls in CppDerivedDynamicClass, PythonDerivedDynamicClass:
+        derived = cls()
+        derived.foobar = 100
+        assert derived.foobar == 100
 
-    assert cstats.alive() == 1
-    del derived
-    assert cstats.alive() == 0
+        assert cstats.alive() == 1
+        del derived
+        assert cstats.alive() == 0
 
 
 def test_cyclic_gc():