Flake check demonstration

flake.lock

{
  "nodes": {
    "nixpkgs": {
      "locked": {
        "lastModified": 1659633244,
        "narHash": "sha256-N+AXQHIyRO5D9T8xrTM+igkqJ90MKsftQJnfW5YNxac=",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "0b04c001b3c2e5009b688a15ae5903aa2293bc7c",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "root": {
      "inputs": {
        "nixpkgs": "nixpkgs"
      }
    }
  },
  "root": "root",
  "version": 7
}

flake.nix

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs";
  };
  outputs = { self, nixpkgs, ... }: rec {
    nixosModules = {
      # with this uncommented, the system build succeeds, but ''nix flake check'' fails
      testModule = ./module.nix;
      
      # with this uncommented, the system build fails.
      #testModule = import ./module.nix;
    };
    nixosConfigurations = {
      doubleImportTest = nixpkgs.lib.nixosSystem {
        system = "x86_64-linux";
        modules = [
          nixosModules.testModule
          nixosModules.testModule # in a real-world situation this would typically be an indirect import
          ({pkgs, ...}: {
            boot.isContainer = true;
            services.testModuleWorks.enable = true;
            system.stateVersion = "22.05";
          })
        ];
      };
    };
  };
}

module.nix

{pkgs, lib, config, ...}: with lib; let cfg = config.services.testModuleWorks; in {
  options.services.testModuleWorks.enable = mkEnableOption "test module";
  config = mkIf cfg.enable {
    systemd.services.testModuleWorks = {
      serviceConfig.ExecStart = "${pkgs.coreutils}/bin/sleep infinity";
    };
  };
}