Bug report

C++20 added support for designated initializers and fails to compile if you mix named and unnamed initializers.

For demonstration I'll use the example from the documentation section "Defining Extension Types: Tutorial" with.tp_doc removed fromCustomType so it builds on Python 3.10.

When building with C++ compiler in C++20 mode using
g++ $(python3-config --cflags) -std=c++20 -Wall -Wextra -Wno-missing-field-initializers -c pymod.cc
it fails with the following message:

pymod.cc:11:5: error: either all initializer clauses should be designated or none of them should be   11 |     .tp_name = "custom.Custom",      |     ^pymod.cc:20:5: error: either all initializer clauses should be designated or none of them should be   20 |     .m_name = "custom",      |     ^

The problem is the macros that produce non-designated initializers here.

Additional information

The following compiles work flawlessly:
GNU C compiler:
gcc $(python3-config --cflags) -Wall -Wextra -Wno-missing-field-initializers -c pymod.cc

GNU C++ compiler with no C++ version specified:
g++ $(python3-config --cflags) -Wall -Wextra -Wno-missing-field-initializers -c pymod.cc

GNU C++ Compiler in C++17 mode:
g++ $(python3-config --cflags) -std=c++17 -Wall -Wextra -Wno-missing-field-initializers -c pymod.cc

Your environment

• Fedora 36 container withpython-devel,gcc andgcc-c++ installed
• CPython: 3.10.7
• GCC: 12.2.1 20220819 (Red Hat 12.2.1-2)

The issue leading up to this was our build failing in C++20 mode on at least RHEL 7/8/9, Fedora 34 to 37, SLES 12, SLES 15, Debian 10/11, Ubuntu 16 to 22.
So I guess building with pretty much every GNU C++ compiler will break if you turn on C++20.

pymod.cc:

#define PY_SSIZE_T_CLEAN#include <Python.h>typedef struct {    PyObject_HEAD    /* Type-specific fields go here. */} CustomObject;static PyTypeObject CustomType = {    PyVarObject_HEAD_INIT(NULL, 0)    .tp_name = "custom.Custom",    .tp_basicsize = sizeof(CustomObject),    .tp_itemsize = 0,    .tp_flags = Py_TPFLAGS_DEFAULT,    .tp_new = PyType_GenericNew,};static PyModuleDef custommodule = {    PyModuleDef_HEAD_INIT,    .m_name = "custom",    .m_doc = "Example module that creates an extension type.",    .m_size = -1,};PyMODINIT_FUNCPyInit_custom(void){    PyObject *m;    if (PyType_Ready(&CustomType) < 0)        return NULL;    m = PyModule_Create(&custommodule);    if (m == NULL)        return NULL;    Py_INCREF(&CustomType);    if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {        Py_DECREF(&CustomType);        Py_DECREF(m);        return NULL;    }    return m;}

Linked PRs

• gh-99202: Fix extension type from documentation for compiling in C++20 mode #102518