Initializing PyModuleDef object with PyModuleDef_HEAD_INIT.

Python 3.8 documentation: m_base - Always initialize this member to PyModuleDef_HEAD_INIT.

Long-standing (since first github commit in 2015), inconsequential bug.

Also removing inconsequential Py_INCREF(def): PyModule_Create() resets the reference count to 1.

Author: rwgk

include/pybind11/detail/common.h

@@ -308,11 +308,11 @@ extern "C" {
         }
 \endrst */
 #if PY_MAJOR_VERSION >= 3
-#define PYBIND11_MODULE_PY2_VS_PY3_SPECIFIC(name)                              \
+#define PYBIND11_MODULE_DETAIL_CREATE(name)                                    \
         static PyModuleDef mdef;                                               \
         auto m = pybind11::module(PYBIND11_TOSTRING(name), nullptr, &mdef);
 #else
-#define PYBIND11_MODULE_PY2_VS_PY3_SPECIFIC(name)                              \
+#define PYBIND11_MODULE_DETAIL_CREATE(name)                                    \
         auto m = pybind11::module(PYBIND11_TOSTRING(name));
 #endif
 #define PYBIND11_MODULE(name, variable)                                        \
@@ -321,7 +321,7 @@ extern "C" {
     PYBIND11_PLUGIN_IMPL(name) {                                               \
         PYBIND11_CHECK_PYTHON_VERSION                                          \
         PYBIND11_ENSURE_INTERNALS_READY                                        \
-        PYBIND11_MODULE_PY2_VS_PY3_SPECIFIC(name)                              \
+        PYBIND11_MODULE_DETAIL_CREATE(name)                                    \
         try {                                                                  \
             PYBIND11_CONCAT(pybind11_init_, name)(m);                          \
             return m.ptr();                                                    \

include/pybind11/pybind11.h

@@ -857,13 +857,20 @@ class module : public object {
     PYBIND11_OBJECT_DEFAULT(module, object, PyModule_Check)
 
     /// Create a new top-level Python module with the given name and docstring
+#if PY_MAJOR_VERSION >= 3
     explicit module(const char *name, const char *doc = nullptr, PyModuleDef *def = nullptr) {
         if (!def) def = new PyModuleDef();
-        std::memset(def, 0, sizeof(PyModuleDef));
-        def->m_name = name;
-        def->m_doc = options::show_user_defined_docstrings() ? doc : nullptr;
-        def->m_size = -1;
-        Py_INCREF(def);
+        def = new (def) PyModuleDef {  // Placement new (not an allocation).
+            /* m_base */     PyModuleDef_HEAD_INIT,
+            /* m_name */     name,
+            /* m_doc */      options::show_user_defined_docstrings() ? doc : nullptr,
+            /* m_size */     -1,
+            /* m_methods */  nullptr,
+            /* m_slots */    nullptr,
+            /* m_traverse */ nullptr,
+            /* m_clear */    nullptr,
+            /* m_free */     nullptr
+        };
         m_ptr = PyModule_Create(def);
 #else
     explicit module(const char *name, const char *doc = nullptr) {