fix: Fixes compilation failure with MSVC 17.12 (v14.44) and C++20: '_BACKUP_ITERATOR_DEBUG_LEVEL' undeclared when including span header from STL.

Author: loumalouomega

include/pybind11/pybind11.h

@@ -9,6 +9,14 @@
 */
 
 #pragma once
+
+// See Issue #5956. This fixes compilation failure with MSVC 17.12 (v14.44) and C++20: '_BACKUP_ITERATOR_DEBUG_LEVEL' undeclared when including span header from STL.
+#if defined(_MSC_VER)
+#ifndef _BACKUP_ITERATOR_DEBUG_LEVEL
+#define _BACKUP_ITERATOR_DEBUG_LEVEL 0
+#endif
+#endif
+
 #include "detail/class.h"
 #include "detail/dynamic_raw_ptr_cast_if_possible.h"
 #include "detail/exception_translation.h"