Conan on windows claims setting isn't set, it is set

I am trying to port a program from Linux to windows. The program is built with conan.

Currently I run:

conan install . -if build -s build_type=Debug

I get this error:

ERROR: : 'settings.compiler.cppstd' value not defined

I have this in my conan.py:

class ConanFileNeverEngine(ConanFile):
    generators = "pkg_config"
    requires = [
        "eigen/3.4.0",
        "libffi/3.4.2", # Not sure why but without this wayland causes a conflict.
        "wayland/1.19.0",
        "glfw/3.3.4",
        "shaderc/2019.0",
        "freetype/2.10.4",
        "eigen/3.4.0",
        "harfbuzz/2.8.2",
        "vulkan-memory-allocator/2.3.0",
        "gtest/1.10.0",
        "benchmark/1.5.3",
        "tinygltf/2.5.0",
        "stb/20200203",
        "vulkan-headers/1.2.182",
        "vulkan-validationlayers/1.2.182",
        "cereal/1.3.0"
    ]
     
    settings = {
        "os": None,
        "compiler" : None,
        "cppstd": ["20"],
        "build_type" : None}
     ....

I also tried to manually set it:

  def config_options(self):
        # self.output.warn("test")
        self.settings.compiler.cppstd = "20"
        self.settings.compiler.runtime = "dynamic"
        if os_info.is_linux:
            if os_info.linux_distro == 'ubuntu':
                window_system = os.environ['XDG_SESSION_TYPE']
                if window_system == 'wayland':
                    self.output.error("System is using wayland, switch to x11.")

I get the exact same error.

I don't understand I AM setting the value.

Solution

Settings are external, project wide configuration, they cannot be defined or assigned values in conanfile.py files.

Settings are defined in your profile, like the "default", you can see it printed when you type conan install, something like:

Configuration:
[settings]
arch=x86_64
arch_build=x86_64
build_type=Release
compiler=Visual Studio

You can check that your defined value is not there, because you are trying to define its value way later.

So you can:

Define your compiler.cppstd in one profile file. The "default" one, or your own custom profile (you can share profiles with conan config install command)
Note you can set settings per package, with mypkg:compiler.cppstd=xxx if necessary, typically as something exceptional, as settings are intended to be project-wide
Pass it in command line -s compiler.cppstd=xxx

The reason for this is that "recipes/conanfile" describe how things are done, but they are generic, they should work for any configuration. Specific configuration values, like cppstd or used compiler then must be external to recipes.