AppleClang identification dependes on CMP0025

Identification of AppleClang compiler in CMAKE_CXX_COMPILER_ID depends on CMP0025 being set to NEW.

However the default behavior is OLD

Unlike most policies, CMake version 3.0.2 does not warn by default when this policy is not set and simply uses OLD behavior. See documentation of the CMAKE_POLICY_WARNING_CMP0025 variable to control the warning.

To help detect this issue cmake-conan should enable the CMAKE_POLICY_WARNING_CMP0025 flag and/or check the value of CMP0025 policy on Mac builds. Also note that this policy must be set before the project declaration.

Otherwise we just get this error

ERROR: Invalid setting '7.0' is not a valid 'settings.compiler.version' value.
Possible values are ['3.3', '3.4', '3.5', '3.6', '3.7', '3.8', '3.9']

because it is identified as Clang instead of AppleClang

[memsharded]

Done it. I have also added CI with travis-ci for both Linux and OSX, and it seems the approach is running fine. A message outputs:

- Conan: APPLE and Clang detected. Assuming apple-clang compiler. Set CMP0025 to avoid it

It is already merged in master, I am going to tag this too. Please try it and report, re-open issue if something fails, or have any question. Thanks!

[memsharded]

Hi @kolis

I see, thanks! The only issue is that I think that it is not possible for conan.cmake to set it. Note how it is used:

cmake_minimum_required(VERSION 2.8)
project(conan_wrapper CXX)
include(conan.cmake)

but as you said, and the docs clearly state, the policy MUST be set before the project declaration:

cmake_policy(SET CMP0025 NEW)
cmake_minimum_required(VERSION 2.8)
project(conan_wrapper CXX)
include(conan.cmake)

There is nothing in the conan.cmake file we provide that could make cmake understand that it is the AppleClang compiler. I guess the only way is to document it, any other idea or suggestion?

As a side note, I have to admit that I am not a big fan of this cmake-conan project, because of this kind of issues, I think it is much better to call conan directly, or to have python scripts that orchestrate the build, calling the necessary tools (conan, cmake, or others), much better scripting in python than having to script in cmake. There are already 300 lines of cmake code, just to avoid a single direct conan install command. Just my personal opinion.