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 from CustomType 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 with python-devel, gcc and gcc-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_FUNC
PyInit_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