Cross compiling from macOS to Emscripten fails because of automatically added -stdlib=libc++

[sascha]

Environment Details

• Operating System+version: macOS 10.15.5
• Compiler+version: Apple clang version 11.0.3 and Emscripten 1.39.19
• Conan version: 1.27.0
• Python version: 3.7.7

Problem description

I'm not quite sure if this is a bug, a missing feature or a configuration error, but the issue is somewhat related to issue #2856. Just like Android, the Emscripten compiler does not need -stdlib=libc++. I could probably get around this issue by specifying compiler.libcxx=c++_static in the host profile in which case the conan_set_libcxx macro in the generated CMake file would not add the -stdlib=libc++ flag, however this is only a valid option when setting the compiler to clang. On the other hand I can't set the compiler to clang, because the highest available version number is 10 while the latest version number of apple-clang is 11. This mismatch will throw an error when installing packages, because Emscripten's toolchain file automatically sets the compiler version to that of the system clang.

Ideally I'd need a way to remove the compiler.libcxx setting when the host os is Emscripten. Is there a way to do this?

• I've read the CONTRIBUTING guide.

[jgsogo]

Hi @sascha ! Can you share with us the Emscripten recipe you are using? or how are you populating the environment before running Conan to compile your library?

[sascha]

Hi @jgsogo, sure, happy to.

I'm installing my Conan deps with

conan install . -if _builds/cesdk/wasm32-unknown-emscripten/debug -s build_type=Debug --build=missing -pr:h=conan/profiles/emscripten -pr:b=default

This is my emscripten profile:

include(default)

[settings]
os=Emscripten
arch=wasm

[build_requires]
emsdk_installer/1.39.19@imgly/stable

and this is my default profile:

[settings]
os=Macos
os_build=Macos
arch=x86_64
arch_build=x86_64
compiler=apple-clang
compiler.version=11.0
compiler.libcxx=libc++
build_type=Release

The emsdk_installer/1.39.19@imgly/stable is pretty much the same as https://github.com/bincrafters/conan-emsdk_installer except that it's updated to the latest version of Emscripten and the lines at https://github.com/bincrafters/conan-emsdk_installer/blob/testing/1.39.13/conanfile.py#L13-L16 have been replaced with a simple settings = "os", "arch" to support the newer host/build profiles.

One of my dependencies is gtest/1.10.0 for which I run into the following error when running conan install with above setup:

clang++: error: argument unused during compilation: '-stdlib=libc++' [-Werror,-Wunused-command-line-argument]

The emscripten compiler frontend is basically a drop-in replacement for clang (https://emscripten.org/docs/tools_reference/emcc.html). Would it make sense to introduce a new compiler setting?

[lkotsonis]

Any update on this? I bumped into the same issue

[jgsogo]

I've been playing with emscripten these days (I'm using the recipe under development here: conan-io/conan-center-index#6163). With the build-requires, I'm able to build corrade (conan-io/conan-center-index#7410) and magnum packages (conan-io/conan-center-index#7412) with some modifications. It also works for magnum-integration, magnum-extras,... those are a bunch of packages.

I'm still working on several pull-requests for different packages, but so far it is working for them: conan-io/conan-center-index#6292.

(All of this, using the latest Conan version)