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

Author: vstinner

Include/pymacro.h

@@ -199,19 +199,23 @@
         } 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(__STRICT_ANSI__) && !defined(__cplusplus))
 #define Py_ARRAY_LENGTH(array) \
     (sizeof(array) / sizeof((array)[0]) \
      + Py_BUILD_ASSERT_EXPR(!__builtin_types_compatible_p(typeof(array), \

Lib/test/test_cext/extension.c

@@ -83,6 +83,7 @@ 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));
@@ -134,6 +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;
 }
 

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.