[3.15] gh-158810: Fix Py_ARRAY_LENGTH() macro on C++ (#159315)

* gh-158810: Fix Py_ARRAY_LENGTH() macro on C++ (#158812)

(cherry picked from commit b2bfbce)

* gh-158810: Use _Py_TYPEOF in Py_ARRAY_LENGTH() (#158838)

Fix the compiler warning "extension used" when building test_cext
with clang.

* test_cext no longer overrides the CC environment variable if it's
  already defined.
* test_cext now also logs __STRICT_ANSI__ macro.

(cherry picked from commit 85c03d4)

* Add Py_SETREF/XSETREF tests to test_cext

Author: vstinner

Include/pymacro.h

@@ -174,23 +174,27 @@
         } while(0)
 #endif
 
-/* Get the number of elements in a visible array
-
-   This does not work on pointers, or arrays declared as [], or function
-   parameters. With correct compiler support, such usage will cause a build
-   error (see Py_BUILD_ASSERT_EXPR).
-
-   Written by Rusty Russell, public domain, http://ccodearchive.net/
-
-   Requires at GCC 3.1+ */
-#if (defined(__GNUC__) && !defined(__STRICT_ANSI__) && \
-    (((__GNUC__ == 3) && (__GNUC_MINOR__ >= 1)) || (__GNUC__ >= 4)))
-/* Two gcc extensions.
-   &a[0] degrades to a pointer: a different type from an array */
+// Get the number of elements in a visible array.
+//
+// This does not work on pointers, or arrays declared as [], or function
+// parameters. With correct compiler support, such usage will cause a build
+// error (see Py_BUILD_ASSERT_EXPR).
+//
+// Written by Rusty Russell, public domain, http://ccodearchive.net/
+//
+// Require GCC 4 (it works on GCC 3.1).
+//
+// Two GCC extensions: &a[0] degrades to a pointer, a different type from an
+// array.
+//
+// gh-158810: Do not use __builtin_types_compatible_p() in strict C ANSI mode
+// and on C++.
+#if (defined(__GNUC__) && __GNUC__ >= 4 && defined(_Py_TYPEOF) \
+        && !defined(__STRICT_ANSI__) && !defined(__cplusplus))
 #define Py_ARRAY_LENGTH(array) \
     (sizeof(array) / sizeof((array)[0]) \
-     + Py_BUILD_ASSERT_EXPR(!__builtin_types_compatible_p(typeof(array), \
-                                                          typeof(&(array)[0]))))
+     + Py_BUILD_ASSERT_EXPR(!__builtin_types_compatible_p(_Py_TYPEOF(array), \
+                                                          _Py_TYPEOF(&(array)[0]))))
 #else
 #define Py_ARRAY_LENGTH(array) \
     (sizeof(array) / sizeof((array)[0]))

Lib/test/test_cext/extension.c

@@ -82,6 +82,8 @@ static PyObject *
 test_macros(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
 {
     PyObject *obj, *dict;
+    PyObject *slots[1];
+    int small_array[] = {2, 5, 7};
 
     // test Py_BUILD_ASSERT() and Py_BUILD_ASSERT_EXPR()
     Py_BUILD_ASSERT(sizeof(int) == sizeof(unsigned int));
@@ -97,17 +99,32 @@ test_macros(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
     Py_CLEAR(obj);
     assert(obj == _Py_NULL);
 
+    // gh-157649: Test Py_CLEAR() on an array
+    slots[0] = Py_None;
+    Py_CLEAR(slots[0]);
+    assert(slots[0] == _Py_NULL);
+
 #ifndef Py_LIMITED_API
     // Test Py_SETREF(): use typeof()/__typeof__() if available, or memcpy()
     obj = Py_None;
     Py_SETREF(obj, _Py_NULL);
     assert(obj == _Py_NULL);
 
+    // gh-157649: Test Py_SETREF() on an array
+    slots[0] = Py_None;
+    Py_SETREF(slots[0], _Py_NULL);
+    assert(slots[0] == _Py_NULL);
+
     // Test Py_XSETREF(): use typeof()/__typeof__() if available, or memcpy()
     obj = Py_None;
     Py_XSETREF(obj, _Py_NULL);
     assert(obj == _Py_NULL);
-#endif
+
+    // gh-157649: Test Py_XSETREF() on an array
+    slots[0] = Py_None;
+    Py_XSETREF(slots[0], _Py_NULL);
+    assert(slots[0] == _Py_NULL);
+#endif  // !Py_LIMITED_API
 
     // Test that Py_BEGIN_CRITICAL_SECTION is available
     dict = PyDict_New();
@@ -118,6 +135,9 @@ test_macros(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
     Py_END_CRITICAL_SECTION();
     Py_DECREF(dict);
 
+    // Test Py_ARRAY_LENGTH()
+    assert(Py_ARRAY_LENGTH(small_array) == 3);
+
     Py_RETURN_NONE;
 }
 

Lib/test/test_cext/setup.py

@@ -155,7 +155,7 @@ def main():
     # option emits a C++ compiler warning. Remove "-std11" option from the
     # CC command.
     cmd = (sysconfig.get_config_var('CC') or '')
-    if cmd is not None:
+    if cmd is not None and 'CC' not in os.environ:
         if support.MS_WINDOWS:
             std_prefix = '/std'
         else:

Misc/NEWS.d/next/C_API/2026-10-05-01-09-32.gh-issue-158810.F1p-PK.rst

@@ -0,0 +1,2 @@
+Fix :c:macro:`Py_ARRAY_LENGTH()` macro on C++: do not use
+``__builtin_types_compatible_p()`` on C++. Patch by Victor Stinner.