gh-91321: Fix compatibility with C++ older than C++11 (#93784) (#93802)

* Fix the compatibility of the Python C API with C++ older than C++11.
* _Py_NULL is only defined as nullptr on C++11 and newer.

(cherry picked from commit 4caf5c2753f1aa28d6f4bc1aa377975fd2a62331)

* test_cppext now builds the C++ extension with setuptools.
* Add @test.support.requires_venv_with_pip.

(cherry picked from commit ca0cc9c433830e14714a5cc93fb4e7254da3dd76)

1 files changed, 51 insertions, 0 deletions

diff --git a/Lib/test/setup_testcppext.py b/Lib/test/setup_testcppext.py
new file mode 100644
index 0000000..a288dbd
--- /dev/null
+++ b/Lib/test/setup_testcppext.py
@@ -0,0 +1,51 @@
+# gh-91321: Build a basic C++ test extension to check that the Python C API is
+# compatible with C++ and does not emit C++ compiler warnings.
+import sys
+from test import support
+
+from setuptools import setup, Extension
+
+
+MS_WINDOWS = (sys.platform == 'win32')
+
+
+SOURCE = support.findfile('_testcppext.cpp')
+if not MS_WINDOWS:
+    # C++ compiler flags for GCC and clang
+    CPPFLAGS = [
+        # gh-91321: The purpose of _testcppext extension is to check that building
+        # a C++ extension using the Python C API does not emit C++ compiler
+        # warnings
+        '-Werror',
+        # Warn on old-style cast (C cast) like: (PyObject*)op
+        '-Wold-style-cast',
+        # Warn when using NULL rather than _Py_NULL in static inline functions
+        '-Wzero-as-null-pointer-constant',
+    ]
+else:
+    # Don't pass any compiler flag to MSVC
+    CPPFLAGS = []
+
+
+def main():
+    cppflags = list(CPPFLAGS)
+    if '-std=c++03' in sys.argv:
+        sys.argv.remove('-std=c++03')
+        std = 'c++03'
+        name = '_testcpp03ext'
+    else:
+        # Python currently targets C++11
+        std = 'c++11'
+        name = '_testcpp11ext'
+
+    cppflags = [*CPPFLAGS, f'-std={std}']
+    cpp_ext = Extension(
+        name,
+        sources=[SOURCE],
+        language='c++',
+        extra_compile_args=cppflags)
+    setup(name=name, ext_modules=[cpp_ext])
+
+
+if __name__ == "__main__":
+    main()