mesonbuild/meson

MSVC: /Zc:__cplusplus not set when --backend=vs

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#14,469 opened on Apr 10, 2025

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backend:visualstudiobuggood first issue

Description

Describe the bug When configuring with MSVC, /Zc:__cplusplus is not set when using --backend=vs, but it is set when using --backend=ninja.

To Reproduce Please include your meson.build files, preferably as a minimal toy example showing the issue. You may need to create simple source code files (don't include private/proprietary code).

meson.build:

project('zccplusplus', 'cpp', meson_version: '>= 1.0',
    default_options:['cpp_std=vc++14', 'buildtype=debug', 'b_vscrt=static_from_buildtype', 'b_ndebug=if-release'])

static_library('zccplusplus', 'zctest.cpp')

zctest.cpp:

#if __cplusplus == 199711L
#error Zc:__cplusplus flag not set
#endif

To reproduce: meson setup --backend=vs vbuild meson compile -C vbuild

The error is not shown if --backend=ninja is used instead.

Expected behavior /Zc:__cplusplus should be consistently set for every back-end when configuring with MSVC.

system parameters

  • Is this a cross build or just a plain native build (for the same computer)?

Native

  • what operating system (e.g. MacOS Catalina, Windows 10, CentOS 8.0, Ubuntu 18.04, etc.)

Windows 11, with MSVC 17.13.6

  • what Python version are you using e.g. 3.8.0

The one bundled with the meson.msi installer.

  • what meson --version

1.7.1

  • what ninja --version if it's a Ninja build

1.12.1 (but it's the --backend=vs that is broken)

Not urgent, can be easily worked around by adding add_project_arguments('/Zc:__cplusplus', language: 'cpp') (because ninja isn't hurt by the redundant setting).

Contributor guide

Tech stack
pythoncpp
Domain
build system
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic understanding of Meson build systemFamiliarity with MSVC compiler flags
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
Investigate the Meson source code, specifically the Visual Studio backend (likely in mesonbuild/backend/vs.py), to find where compiler flags are set. Compare with the Ninja backend to see how /Zc: cplusplus is added. Add the flag to the VS backend as well, ensuring it is consistently applied. Check if any conditional logic is needed based on compiler version or standard.