Conan not respecting compiler.libcxx abi setting

Question

I have the following conan profile:

$ conan profile show default
Configuration for profile default:

[settings]
os=Linux
os_build=Linux
arch=x86_64
arch_build=x86_64
compiler=gcc
compiler.version=11
compiler.cppstd=20
compiler.libcxx=libstdc++
build_type=Release
[options]
[conf]
[build_requires]
[env]

Note that compiler.libcxx is the old ABI (libstdc++) and not the new ABI (libstdc++11). This is intentional. Also I'm specifying cppstd=20.

I then made a custom conanfile for {fmt}, which tries to package up fmt 9.0. That looks like, in its entirety:

from conans import ConanFile, CMake, tools
from conan.tools.cmake import CMakeToolchain, cmake_layout

class ConanFmt(ConanFile):
    name = "fmt"
    version = "9.0.0"

    settings = "os", "compiler", "cppstd", "build_type", "arch"

    def source(self):
        git = tools.Git(folder="fmt")
        git.clone("https://github.com/fmtlib/fmt.git", "9.0.0", shallow=True)

    def layout(self):
        cmake_layout(self)

    def generate(self):
        tc = CMakeToolchain(self)
        tc.generate()

    def build(self):
        cmake = CMake(self)
        cmake.definitions["FMT_DOC"] = False
        cmake.definitions["FMT_TEST"] = False
        cmake.definitions["FMT_INSTALL"] = True
        cmake.definitions["CMAKE_EXPORT_COMPILE_COMMANDS"] = True
        cmake.configure(source_folder="fmt")
        cmake.build()

    def package(self):
        cmake = CMake(self)
        cmake.install()

    def package_info(self):
        self.cpp_info.libs = ["fmt"]

When I try to go ahead and package this up, like say:

$ conan create . demo/testing -s build_type=Release

This does actually go ahead and clone the {fmt} repo, and correctly build and package up the contents of the library.

Except that:

there is no _GLIBCXX_USE_CXX11_ABI definition set (despite what this page) says.
{fmt} was compiled with C++11, not C++20
... but it was built with Release (-O3 -DNDEBUG)

Conan does appear to provide variables like CONAN_CMAKE_CXX_STANDARD=20, CONAN_STD_CXX_FLAG=-std=c++2a, and CONAN_LIBCXX=libstdc++. But {fmt}'s build doesn't actually use those, so that isn't very useful.

How do I actually get conan to propagate through these fields properly? Is that something that must be explicitly specified in the conanfile.py (even if it's in the profile)?

Conan not respecting compiler.libcxx abi setting

0 Answers