[BUG]: Compilation failure with MSVC 17.12 (v14.44) and C++20: '_BACKUP_ITERATOR_DEBUG_LEVEL' undeclared

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What version (or hash if on master) of pybind11 are you using?

3.0.1

Problem description

This is a well-structured GitHub issue template you can use. It specifically targets the MSVC 17.12 incompatibility with the iterator debugging macros found in the Kratos core.

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GitHub Issue Title:

Compilation failure with MSVC 17.12 (v14.44) and C++20: '_BACKUP_ITERATOR_DEBUG_LEVEL' undeclared

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Description

When compiling Kratos Multiphysics, see https://github.com/KratosMultiphysics/Kratos/pull/14105, with the last pybind11 release on Windows using Visual Studio 2022 version 17.12 (MSVC 14.44) with the std:c++20 flag, the build fails in several python binding files due to a macro conflict in the MSVC Standard Library headers.

The error appears to originate from the <span> header in the MSVC STL, which seems unable to resolve the custom _BACKUP_ITERATOR_DEBUG_LEVEL macro used by Kratos to manage iterator debugging states.

Environment

• OS: Windows 10/11
• Compiler: MSVC 14.44.35207 (Visual Studio 2022 17.12)
• C++ Standard: C++20
• Python version: 3.10.11
• Build Type: Release

Error Log

C:\Program Files (x86)\...\include\span(179): error C2065: '_BACKUP_ITERATOR_DEBUG_LEVEL' : undeclared identifier
C:\Program Files (x86)\...\include\span(179): note: the template instantiation context (the oldest) is
C:\Program Files (x86)\...\include\span(31): note: during compilation of class template 'std::_Span_iterator'
C:\Program Files (x86)\...\include\span(179): error C3861: '_BACKUP_ITERATOR_DEBUG_LEVEL': identifier not found

Steps to Reproduce

1. Use Visual Studio 2022 updated to version 17.12.
2. Configure Kratos with CMake using -DCMAKE_CXX_STANDARD=20.
3. Attempt to build the Kratos Core.

Potential Workaround

Adding in the header of <pybind11/pybind11.h>:

#if defined(_MSC_VER)
#ifndef _BACKUP_ITERATOR_DEBUG_LEVEL
#define _BACKUP_ITERATOR_DEBUG_LEVEL 0
#endif
#endif

Reproducible example code

(Sorry, but it is a full codebase, so I cannot isolate a snippet)

Is this a regression? Put the last known working version here if it is.

Not a regression

Start with pybind11/pybind11.h and the MSVC error context, then reproduce the CMake C++20 build described for Kratos PR 14105 using MSVC 14.44. Done means the affected build compiles without the _BACKUP_ITERATOR_DEBUG_LEVEL error in the stated environment.