gh-129666: Add C11/C++11 to docs and -pedantic-errors to GCC/clang test_c[pp]ext tests

Doc/c-api/intro.rst

@@ -30,6 +30,16 @@ familiar with writing an extension before  attempting to embed Python in a real
 application.
 
 
+Language version compatibility
+==============================
+
+Python's C API is compatible with C11 and C++11 versions of C and C++.
+
+This is a lower limit: the C API does not require features from later
+C/C++ versions.
+You do *not* need to enable your compiler's "c11 mode".
+
+
 Coding standards
 ================
 

Lib/test/test_cext/__init__.py

@@ -38,6 +38,9 @@ def test_build_c11(self):
 
     @unittest.skipIf(support.MS_WINDOWS, "MSVC doesn't support /std:c99")
     def test_build_c99(self):
+        # In public docs, we say C API is compatible with C11. However,
+        # in practice we do maintain C99 compatibility in public headers.
+        # Please ask the C API WG before adding a new C11-only feature.
         self.check_build('_test_c99_cext', std='c99')
 
     @support.requires_gil_enabled('incompatible with Free Threading')

Lib/test/test_cext/extension.c

@@ -58,11 +58,24 @@ _testcext_exec(
     return 0;
 }
 
+// Converting from function pointer to void* has undefined behavior, but
+// works on all known platforms, and CPython's module and type slots currently
+// need it.
+// (GCC doesn't have a narrower category for this than -Wpedantic.)
+_Py_COMP_DIAG_PUSH
+#if defined(__GNUC__)
+#pragma GCC diagnostic ignored "-Wpedantic"
+#elif defined(__clang__)
+#pragma clang diagnostic ignored "-Wpedantic"
+#endif
+
 static PyModuleDef_Slot _testcext_slots[] = {
     {Py_mod_exec, (void*)_testcext_exec},
     {0, NULL}
 };
 
+_Py_COMP_DIAG_POP
+
 
 PyDoc_STRVAR(_testcext_doc, "C test extension.");
 

Lib/test/test_cext/setup.py

@@ -21,6 +21,9 @@
 
         # gh-120593: Check the 'const' qualifier
         '-Wcast-qual',
+
+        # Ask for strict(er) compliance with the standard
+        '-pedantic-errors',
     ]
     if not support.Py_GIL_DISABLED:
         CFLAGS.append(

Lib/test/test_cppext/__init__.py

@@ -29,6 +29,9 @@ def test_build(self):
         self.check_build('_testcppext')
 
     def test_build_cpp03(self):
+        # In public docs, we say C API is compatible with C++11. However,
+        # in practice we do maintain C++03 compatibility in public headers.
+        # Please ask the C API WG before adding a new C++11-only feature.
         self.check_build('_testcpp03ext', std='c++03')
 
     @unittest.skipIf(support.MS_WINDOWS, "MSVC doesn't support /std:c++11")

Lib/test/test_cppext/extension.cpp

@@ -161,11 +161,24 @@ class VirtualPyObject : public PyObject {
 
 int VirtualPyObject::instance_count = 0;
 
+// Converting from function pointer to void* has undefined behavior, but
+// works on all known platforms, and CPython's module and type slots currently
+// need it.
+// (GCC doesn't have a narrower category for this than -Wpedantic.)
+_Py_COMP_DIAG_PUSH
+#if defined(__GNUC__)
+#pragma GCC diagnostic ignored "-Wpedantic"
+#elif defined(__clang__)
+#pragma clang diagnostic ignored "-Wpedantic"
+#endif
+
 PyType_Slot VirtualPyObject_Slots[] = {
     {Py_tp_free, (void*)VirtualPyObject::dealloc},
     {0, _Py_NULL},
 };
 
+_Py_COMP_DIAG_POP
+
 PyType_Spec VirtualPyObject_Spec = {
     /* .name */ STR(MODULE_NAME) ".VirtualPyObject",
     /* .basicsize */ sizeof(VirtualPyObject),
@@ -241,11 +254,20 @@ _testcppext_exec(PyObject *module)
     return 0;
 }
 
+// Need to ignore "-Wpedantic" warnings; see VirtualPyObject_Slots above
+_Py_COMP_DIAG_PUSH
+#if defined(__GNUC__)
+#pragma GCC diagnostic ignored "-Wpedantic"
+#elif defined(__clang__)
+#pragma clang diagnostic ignored "-Wpedantic"
+#endif
+
 static PyModuleDef_Slot _testcppext_slots[] = {
     {Py_mod_exec, reinterpret_cast<void*>(_testcppext_exec)},
     {0, _Py_NULL}
 };
 
+_Py_COMP_DIAG_POP
 
 PyDoc_STRVAR(_testcppext_doc, "C++ test extension.");
 

Lib/test/test_cppext/setup.py

@@ -18,6 +18,14 @@
         # a C++ extension using the Python C API does not emit C++ compiler
         # warnings
         '-Werror',
+
+        # Ask for strict(er) compliance with the standard.
+        '-pedantic-errors',
+
+        # But allow C++11 features for -std=C++03. We use:
+        # - `long long` (-Wno-c++11-long-long)
+        # - comma at end of `enum` lists (no narrower GCC option exists)
+        '-Wno-c++11-extensions',
     ]
 else:
     # MSVC compiler flags