gh-121617: Include <string.h> for Py_CLEAR() macro

If the _Py_TYPEOF macro is not available, include <string.h> to get
memcpy() for the Py_CLEAR() macro. The limited C API version 3.11 and
newer doesn't include <string.h> anymore.

Add a Py_CLEAR() test in test_cext.

Modify also _Py_TYPEOF to use C23 typeof() if available.

Author: vstinner

Include/pyport.h

@@ -567,8 +567,11 @@ extern "C" {
 //
 // Example: _Py_TYPEOF(x) x_copy = (x);
 //
-// The macro is only defined if GCC or clang compiler is used.
-#if defined(__GNUC__) || defined(__clang__)
+// On C23, use typeof(). Otherwise, the macro is only defined
+// if GCC or clang compiler is used.
+#if defined (__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#  define _Py_TYPEOF(expr) typeof(expr)
+#elif defined(__GNUC__) || defined(__clang__)
 #  define _Py_TYPEOF(expr) __typeof__(expr)
 #endif
 

Include/refcount.h

@@ -471,6 +471,9 @@ static inline Py_ALWAYS_INLINE void Py_DECREF(PyObject *op)
  * and so avoid type punning. Otherwise, use memcpy() which causes type erasure
  * and so prevents the compiler to reuse an old cached 'op' value after
  * Py_CLEAR().
+ *
+ * Include <string.h> if _Py_TYPEOF() is not available, since the limited C API
+ * version 3.11 and newer doesn't include it.
  */
 #ifdef _Py_TYPEOF
 #define Py_CLEAR(op) \
@@ -483,6 +486,7 @@ static inline Py_ALWAYS_INLINE void Py_DECREF(PyObject *op)
         } \
     } while (0)
 #else
+#include <string.h>               // memcpy()
 #define Py_CLEAR(op) \
     do { \
         PyObject **_tmp_op_ptr = _Py_CAST(PyObject**, &(op)); \

Lib/test/test_cext/extension.c

@@ -65,6 +65,8 @@ _testcext_exec(
 #endif
     )
 {
+    PyObject *obj;
+
 #ifdef __STDC_VERSION__
     if (PyModule_AddIntMacro(module, __STDC_VERSION__) < 0) {
         return -1;
@@ -75,6 +77,10 @@ _testcext_exec(
     Py_BUILD_ASSERT(sizeof(int) == sizeof(unsigned int));
     assert(Py_BUILD_ASSERT_EXPR(sizeof(int) == sizeof(unsigned int)) == 0);
 
+    // Test Py_CLEAR()
+    obj = NULL;
+    Py_CLEAR(obj);
+
     return 0;
 }
 

Misc/NEWS.d/next/C_API/2026-02-10-14-49-49.gh-issue-121617.57vMqa.rst

@@ -0,0 +1,3 @@
+Fix the :c:macro:`Py_CLEAR` implementation in the limited C API version 3.11
+and newer: include ``<string.h>`` to get the ``memcpy()`` function. Patch by
+Victor Stinner.