[feature] Add an compiler attribute to settings if clang so that conan client & recipes has a chance to disambiguate between windows clang flavors

[SpaceIm]

What is your suggestion?

It's more or less related to #12336, and to many issues conan-center is facing from users building recipes with clang-cl, LLVM clang, MinGW clang etc.

Recipes have sometimes to make decision depending on the compiler:

• raise if not supported
• build with Autotools, CMake, NMake, MSBuild or Meson depending on the compiler (you can do a build with NMake or MSBuild if compiler is clang-cl, but not if it's clang MinGW).
• inject options, flags, env vars
• predict lib name of each component

Currently, it's impossible for a recipe to disambiguate which windows clang flavor a consumer is using, and clang-cl is fundamentally different to MinGW clang. There should be a compiler attribute, or anything else in settings which should be the canonical conan way to express a specific clang flavor on Windows to conan, so that recipes can rely on it for any custom logic (bonus: add helpers like is_clang_cl(), is_mingw() based on settings).

Many conan-center recipes are broken for at least one Windows clang flavor, without any way to fix it without breaking the other.

Have you read the CONTRIBUTING guide?

• I've read the CONTRIBUTING guide

[memsharded]

This topic has been developed in https://blog.conan.io/2022/10/13/Different-flavors-Clang-compiler-Windows.html. The blog post contains specific profiles that define the configuration for the different flavors, clearly dissambiguating between those different flavors. Please refer to it and report about inaccuracies or missing bit from it.

[SpaceIm]

I think I've already mentioned my concerns about these profiles in #12336

In this blog post:

llvm_clang.profile

[settings]
os=Windows
arch=x86_64
build_type=Release
compiler=clang
compiler.version=13
compiler.cppstd=gnu14
compiler.runtime=dynamic
compiler.runtime_type=Release
compiler.runtime_version=v143

vs_clang.profile:

It will use it via the clang-cl.exe compiler driver that is compatible with the MSVC compiler command line arguments. The compiler is still the same internally (the same as LLVM/Clang), just it will accept Visual Studio arguments compatible with msvc cl.exe compiler.

[settings]
os=Windows
arch=x86_64
build_type=Release
compiler=clang
compiler.version=14
compiler.cppstd=gnu14
compiler.runtime=dynamic
compiler.runtime_type=Release
compiler.runtime_version=v143

msys2_clang.profile (and all other mingw flavors which are usually similar for a recipe)

[settings]
os=Windows
arch=x86_64
build_type=Release
compiler=clang
compiler.version=14
compiler.cppstd=gnu14
compiler.libcxx=libc++

How do you disambiguate between these profiles given that:

• LLVM clang & clang-cl can't go in the same build branch in many recipes (because LLVM clang is not a cl-like compiler) => no hope here for a recipe to understand from a profile what is the flavor.
• compiler.libcxx is removed in configure() in case of C library, it's fragile (we can just check that compiler.runtime doesn't exist eventually).

[memsharded]

because LLVM clang is not a cl-like compiler

LLVM/Clang IS the clang-cl compiler. The MSVC internal clang-cl is just a distribution of the LLVM/Clang compiler.
Irrespective of the frontend (clang-cl or pure clang) both are MSVC based, linking with VS runtime, and same binary compatibility. That is the only improvement that would be missing in the blog post, to explicitly opt-in to one front-end or the other, but otherwise, it is very different from the MSys2 one, and using the existence of runtime is the thing that disambiguates between both families.

[SpaceIm]

This frontend difference is important for many recipes (and for conan client also I guess to know if cl like flags can be passed).

[SpaceIm]

Regarding runtime, I think an helper would help: is_mingw() (I've proposed #12678 two months ago).

And if something is added to opt-in the front-end, another helper is_cl_like() would be nice (#12336), and/or is_clang_cl()

I don't think it's obvious currently, otherwise conan team would have been able to provide a robust example in real complex recipes of conan-center where compiler flavors matter.

[SpaceIm]

There is another issue: is_msvc_static_runtime() always returns False when compiler is clang, it may lead to incorrect binaries for clang-cl.