Fix MSCV C++ compilation error of pycore_stackref.h header

Wraps the header in a C file and compile it using a C compiler, which should support designated initializers

Fix issue #160647

build_variables.bzl

@@ -926,6 +926,7 @@ libtorch_python_core_sources = [
     "torch/csrc/dynamo/guards.cpp",
     "torch/csrc/dynamo/utils.cpp",
     "torch/csrc/dynamo/init.cpp",
+    "torch/csrc/dynamo/stackref_bridge.c",
     "torch/csrc/functorch/init.cpp",
     "torch/csrc/fx/node.cpp",
     "torch/csrc/mps/Module.cpp",

torch/csrc/dynamo/cpython_includes.h

@@ -28,6 +28,7 @@
 #include <internal/pycore_stackref.h>
 #elif IS_PYTHON_3_14_PLUS && defined(_WIN32)
 #include <internal/pycore_interpframe_structs.h> // _PyInterpreterFrame
+#include "stackref_bridge.h"
 #endif
 
 #endif
@@ -45,7 +46,7 @@ extern "C" {
 
 #elif IS_PYTHON_3_14_PLUS && defined(_WIN32)
 
-#define F_CODE(x) ((PyCodeObject*)((x)->f_executable.bits))
+#define F_CODE(x) ((PyCodeObject*)Torch_PyStackRef_AsPyObjectBorrow(&x->f_executable))
 #define PREV_INSTR(x) (x)->instr_ptr
 
 #else

torch/csrc/dynamo/stackref_bridge.c

@@ -0,0 +1,19 @@
+// Compile this file as C, not C++.
+
+// Wrap inclusion of pycore_stackref.h inside a pure C file and ensure this file
+// is compiled as C (not C++). This avoids MSVC’s “designated initializers
+// require /std:c++20” error, since pycore_stackref.h uses C99-style designated
+// initializers that are not supported in older C++ standards, but is supported
+// in C.
+
+#define Py_BUILD_CORE
+#include <Python.h>
+#include <internal/pycore_stackref.h>
+#include "stackref_bridge.h"
+#undef Py_BUILD_CORE
+
+
+PyObject* Torch_PyStackRef_AsPyObjectBorrow(void* stackref) {
+  _PyStackRef *sr = (_PyStackRef*)stackref;
+  return PyStackRef_AsPyObjectBorrow(*sr);
+}

torch/csrc/dynamo/stackref_bridge.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#ifdef _WIN32
+
+#ifdef __cplusplus
+extern "C" {
+#endif  // __cplusplus
+
+// Use a void* to avoid exposing the internal _PyStackRef union on this
+// translation unit
+PyObject* Torch_PyStackRef_AsPyObjectBorrow(void* stackref);
+
+#ifdef __cplusplus
+}
+#endif  // #ifdef __cplusplus
+
+#endif  // #ifdef _WIN32