gh-91321: Add basic _testcppext C++ extension

Build a basic C++ test extension to check that the Python C API is
compatible with C++ and does not emit C++ compiler warnings.

* Add Modules/_testcppext.cpp: C++ extension
* Add Lib/test/test_cppext.py: test building the C++ extension.

Author: vstinner

Lib/test/_testcppext.cpp

@@ -0,0 +1,62 @@
+// gh-91321: Very basic C++ test extension to check that the Python C API is
+// compatible with C++ and does not emit C++ compiler warnings.
+
+#include "Python.h"
+
+PyDoc_STRVAR(_testcppext_add_doc,
+"add(x, y)\n"
+"\n"
+"Return the sum of two integers: x + y.");
+
+static PyObject *
+_testcppext_add(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    long i, j;
+    if (!PyArg_ParseTuple(args, "ll:foo", &i, &j)) {
+        return nullptr;
+    }
+    long res = i + j;
+    return PyLong_FromLong(res);
+}
+
+
+static PyMethodDef _testcppext_methods[] = {
+    {"add", _testcppext_add, METH_VARARGS, _testcppext_add_doc},
+    {nullptr, nullptr, 0, nullptr}  /* sentinel */
+};
+
+
+static int
+_testcppext_exec(PyObject *module)
+{
+    if (PyModule_AddIntMacro(module, __cplusplus) < 0) {
+        return -1;
+    }
+    return 0;
+}
+
+static PyModuleDef_Slot _testcppext_slots[] = {
+    {Py_mod_exec, reinterpret_cast<void*>(_testcppext_exec)},
+    {0, NULL}
+};
+
+
+PyDoc_STRVAR(_testcppext_doc, "C++ test extension.");
+
+static struct PyModuleDef _testcppext_module = {
+    PyModuleDef_HEAD_INIT,  // m_base
+    "_testcppext",  // m_name
+    _testcppext_doc,  // m_doc
+    0,  // m_size
+    _testcppext_methods,  // m_methods
+    _testcppext_slots,  // m_slots
+    NULL,  // m_traverse
+    NULL,  // m_clear
+    nullptr,  // m_free
+};
+
+PyMODINIT_FUNC
+PyInit__testcppext(void)
+{
+    return PyModuleDef_Init(&_testcppext_module);
+}

Lib/test/test_cppext.py

@@ -0,0 +1,74 @@
+# 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 os
+import sys
+import unittest
+import warnings
+from test import support
+from test.support import os_helper
+
+with warnings.catch_warnings():
+    warnings.simplefilter('ignore', DeprecationWarning)
+    from distutils.core import setup, Extension
+    # Import sysconfig here to make the DeprecationWarning quiet
+    import distutils.sysconfig
+
+
+MS_WINDOWS = (sys.platform == 'win32')
+
+
+SOURCE = support.findfile('_testcppext.cpp')
+if not MS_WINDOWS:
+    # C++ compiler flags for GCC and clang
+    CPPFLAGS = [
+        # Python currently targets C++11
+        '-std=c++11',
+        # 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',
+    ]
+else:
+    # Don't pass any compiler flag to MSVC
+    CPPFLAGS = []
+
+
+class TestCPPExt(unittest.TestCase):
+    def build(self):
+        cpp_ext = Extension(
+            '_testcppext',
+            sources=[SOURCE],
+            language='c++',
+            extra_compile_args=CPPFLAGS)
+
+        try:
+            try:
+                with (support.captured_stdout() as stdout,
+                      support.swap_attr(sys, 'argv', ['setup.py', 'build_ext'])):
+                    setup(name="_testcppext", ext_modules=[cpp_ext])
+                    return
+            except:
+                # Show output on error
+                print()
+                print(stdout.getvalue())
+                raise
+        except SystemExit:
+            self.fail("Build failed")
+
+    # With MSVC, the linker fails with: cannot open file 'python311.lib'
+    # https://github.com/python/cpython/pull/32175#issuecomment-1111175897
+    @unittest.skipIf(MS_WINDOWS, 'test fails on Windows')
+    def test_build(self):
+        # save/restore os.environ
+        def restore_env(old_env):
+            os.environ.clear()
+            os.environ.update(old_env)
+        self.addCleanup(restore_env, dict(os.environ))
+
+        # Build in a temporary directory
+        with os_helper.temp_cwd():
+            self.build()
+
+
+if __name__ == "__main__":
+    unittest.main()