Last month, I embarked on a new project. I set up a new computer with the latest Debian version, installed my favorites tools, and was all set to code. My first task was to migrate all the repositories from C++14 to C++20. While it might seem as straightforward as updating all the CMakeLists.txt to replace set (CMAKE_CXX_STANDARD 14) with set (CMAKE_CXX_STANDARD 20) reality proved otherwise (and I knew it would).

CMake 3.25 introduced a new variable called SYSTEM. It will help us handle warnings from 3rd party libraries. Let’s see how!

The issue (with a minimal example project)

Here is a minimal example project to reproduce the issue. It simply prints something with the great fmt library:

#include <fmt/core.h>

int main()
{
    fmt::print("The answer is {}", 42);
}

The library is fetched from GitHub thanks to CMake’s FetchContent module:

cmake_minimum_required(VERSION 3.25)
project(CMake_SYSTEM)

# Set C++ standard
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

# Fetch fmt from GitHub
include(FetchContent)

FetchContent_Declare(
        fmt
        GIT_REPOSITORY https://github.com/fmtlib/fmt.git
        GIT_TAG 9.1.0
)

FetchContent_MakeAvailable(fmt)

# Create executable
add_executable(${PROJECT_NAME} main.cpp)

target_compile_options(${PROJECT_NAME} PRIVATE -Wall -Wextra -pedantic -Wswitch-enum)

target_link_libraries(${PROJECT_NAME} PRIVATE fmt)

The project can be built with: