Cython cannot use "pycore_frame.h" in Py3.14a4

Bug report

Bug description:

It's this time of the year again. As discussed before (e.g. https://github.com/python/cpython/issues/123747), Cython #includes CPython's pycore_frame.h to get access to certain frame features, e.g. their integration into tracebacks.

I noticed that Cython generated code doesn't compile in CPython 3.14 alpha with the following kind of errors:

…\Python\3.14.0-alpha.4\x64\include\internal\pycore_stackref.h(278): error C7555: use of designated initializers requires at least '/std:c++20'
…\Python\3.14.0-alpha.4\x64\include\internal/pycore_frame.h(196): error C7555: use of designated initializers requires at least '/std:c++20'
…\Python\3.14.0-alpha.4\x64\include\internal/pycore_frame.h(196): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
…\Python\3.14.0-alpha.4\x64\include\internal/pycore_frame.h(378): error C7555: use of designated initializers requires at least '/std:c++20'
…\Python\3.14.0-alpha.4\x64\include\internal/pycore_frame.h(378): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
…\Python\3.14.0-alpha.4\x64\include\internal/pycore_frame.h(379): error C7555: use of designated initializers requires at least '/std:c++20'
…\Python\3.14.0-alpha.4\x64\include\internal/pycore_frame.h(379): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax

This is from one of our Windows CI runs. Py3.14a4 seems to be what Github Actions currently provides. Cython code expects C99 and something close to (but not necessarily as complete as) C++11 as compiler standards. It targets both C and C++, depending on user needs.

The dependency on pycore_stackref.h was apparently added in https://github.com/python/cpython/pull/118450:

22b0de2755e [2024-06-27 03:10:43 +0800] GitHub | gh-117139: Convert the evaluation stack to stack refs (#118450)

diff --git a/Include/internal/pycore_frame.h b/Include/internal/pycore_frame.h
index bab92c771a7..1e0368faa5b 100644
--- a/Include/internal/pycore_frame.h
+++ b/Include/internal/pycore_frame.h
@@ -11,6 +11,7 @@ extern "C" {
 #include <stdbool.h>
 #include <stddef.h>               // offsetof()
 #include "pycore_code.h"          // STATS
+#include "pycore_stackref.h"      // _PyStackRef
 
 /* See Objects/frame_layout.md for an explanation of the frame stack
  * including explanation of the PyFrameObject and _PyInterpreterFrame
@@ -67,7 +68,7 @@ typedef struct _PyInterpreterFrame {
     uint16_t return_offset;  /* Only relevant during a function call */
     char owner;
     /* Locals and stack */
-    PyObject *localsplus[1];
+    _PyStackRef localsplus[1];
 } _PyInterpreterFrame;
 
 #define _PyInterpreterFrame_LASTI(IF) \

What can we do to resolve this?

CPython versions tested on:

3.14

Operating systems tested on:

Windows

Linked PRs

• gh-131249
• gh-131252
• gh-131396

Start with Include/internal/pycore_frame.h and Include/internal/pycore_stackref.h, focusing on the designated initializers and the C++ standard errors reported by the Windows build. Review linked PRs gh-131249, gh-131252, and gh-131396 before making changes. Done means generated Cython code using these headers compiles on Windows with the stated C99 and C++11-era requirements.