gh-91321: Add _Py_NULL macro

[vstinner]

Fix C++ compiler warnings: "zero as null pointer constant"
(clang -Wzero-as-null-pointer-constant).

• Add the _Py_NULL macro used by static inline functions to use
  nullptr in C++.
• Replace NULL with nullptr in _testcppext.cpp.

[vstinner]

I already had this issue and fixed it in pythoncapi-compat. I prepared a PR for pythoncapi-compat to reuse Python 3.11 _Py_NULL macro (this PR): python/pythoncapi-compat#36

[vstinner]

Currently, with gcc -Wzero-as-null-pointer-constant, there is no C++ compiler warning when including <Python.h>. But using clang -Wzero-as-null-pointer-constant, I get many warnings (errors). A few examples:

$ CC=clang ./python -m test -v test_cppext
(...)
In file included from /home/vstinner/python/main/Lib/test/_testcppext.cpp:7:
In file included from /home/vstinner/python/main/Include/Python.h:44:
/home/vstinner/python/main/Include/object.h:590:15: error: zero as null pointer constant [-Werror,-Wzero-as-null-pointer-constant]
    if (op != NULL) {
              ^~~~
              nullptr
/usr/lib64/clang/13.0.0/include/stddef.h:84:18: note: expanded from macro 'NULL'
#    define NULL __null
                 ^
In file included from /home/vstinner/python/main/Lib/test/_testcppext.cpp:7:
In file included from /home/vstinner/python/main/Include/Python.h:44:
/home/vstinner/python/main/Include/object.h:600:15: error: zero as null pointer constant [-Werror,-Wzero-as-null-pointer-constant]
    if (op != NULL) {
              ^~~~
              nullptr
/usr/lib64/clang/13.0.0/include/stddef.h:84:18: note: expanded from macro 'NULL'
#    define NULL __null
                 ^
In file included from /home/vstinner/python/main/Lib/test/_testcppext.cpp:7:
In file included from /home/vstinner/python/main/Include/Python.h:51:
In file included from /home/vstinner/python/main/Include/unicodeobject.h:1042:
/home/vstinner/python/main/Include/cpython/unicodeobject.h:645:42: error: zero as null pointer constant [-Werror,-Wzero-as-null-pointer-constant]
    if (_PyASCIIObject_CAST(op)->wstr == NULL) {
                                         ^~~~
                                         nullptr
/usr/lib64/clang/13.0.0/include/stddef.h:84:18: note: expanded from macro 'NULL'
#    define NULL __null
                 ^
(...)
Tests result: FAILURE

[vstinner]

I also got these errors on the CI of the datatabl project: see h2oai/datatable#3231

[neutrinoceros]

This breaks C-extensions builds with , e.g., -std=c++03, because nullptr is only defined since C++11
Is it desired that Python 3.11 de facto requires C++11 or above ?

[vstinner]

In Python 3.11, I added test_cppext which checks that the Python C API is compatible with C++11. Do you care about older C++ versions? What is your OS (name and version) and C++ compiler (name and version)?

[neutrinoceros]

Actually we're explicitly requiring -std=c++03 in a couple Cython files.
This is probably something we can (and should) update on our side, I just wanted to check if the breaking change was intentional :)

[vstinner]

I wrote #93784 to fix compatibility with C++ older than C++11.

[vstinner]

I just wanted to check if the breaking change was intentional :)

It's not intentional. My initial concern was to prevent C++ compiler warnings.

[neutrinoceros]

Awesome, thank you very much !

[vstinner]

Python 3.11 is now fixed:

• [3.11] gh-91321: Fix compatibility with C++ older than C++11 (#93784) #93802
• ef591cf