Merge pull request #46341 from obsidiansystems/fix-46320

nixpkgs module: Fix defaulting of `localSystem` and `system`

nixos/lib/eval-config.nix

···
     _file = ./eval-config.nix;
     key = _file;
     config = {
-      nixpkgs.localSystem = lib.mkDefault { inherit system; };
+      # Explicit `nixpkgs.system` or `nixpkgs.localSystem` should override
+      # this.  Since the latter defaults to the former, the former should
+      # default to the argument. That way this new default could propagate all
+      # they way through, but has the last priority behind everything else.
+      nixpkgs.system = lib.mkDefault system;
       _module.args.pkgs = lib.mkIf (pkgs_ != null) (lib.mkForce pkgs_);
     };
   };

nixos/modules/misc/nixpkgs.nix

···
     pkgs = mkOption {
       defaultText = literalExample
         ''import "''${nixos}/.." {
-            inherit (config.nixpkgs) config overlays localSystem crossSystem;
+            inherit (cfg) config overlays localSystem crossSystem;
           }
         '';
       default = import ../../.. {
-        localSystem = { inherit (cfg) system; } // cfg.localSystem;
-        inherit (cfg) config overlays crossSystem;
+        inherit (cfg) config overlays localSystem crossSystem;
       };
       type = pkgsType;
       example = literalExample ''import <nixpkgs> {}'';
···
 
     localSystem = mkOption {
       type = types.attrs; # TODO utilize lib.systems.parsedPlatform
-      default = { system = builtins.currentSystem; };
+      default = { inherit (cfg) system; };
       example = { system = "aarch64-linux"; config = "aarch64-unknown-linux-gnu"; };
+      # Make sure that the final value has all fields for sake of other modules
+      # referring to this. TODO make `lib.systems` itself use the module system.
+      apply = lib.systems.elaborate;
       defaultText = literalExample
         ''(import "''${nixos}/../lib").lib.systems.examples.aarch64-multiplatform'';
       description = ''
···
     system = mkOption {
       type = types.str;
       example = "i686-linux";
+      default = { system = builtins.currentSystem; };
       description = ''
         Specifies the Nix platform type on which NixOS should be built.
         It is better to specify <code>nixpkgs.localSystem</code> instead.
···
         </programlisting>
         See <code>nixpkgs.localSystem</code> for more information.
 
+        Ignored when <code>nixpkgs.localSystem</code> is set.
         Ignored when <code>nixpkgs.pkgs</code> is set.
       '';
     };