Add assertion for needed upgrade to home-manager 20.03

modules/home-manager.nix

@@ -7,6 +7,9 @@ with lib;
 let
   cfg = config.home-manager;
 
+  tryEval = builtins.tryEval <home-manager/modules/modules.nix>;
+  assertion = tryEval.success && builtins.functionArgs (import tryEval.value) ? useNixpkgsModule;
+
   extendedLib = import <home-manager/modules/lib/stdlib-extended.nix> pkgs.lib;
 
   hmModule = types.submoduleWith {
@@ -49,7 +52,10 @@ in
       };
 
       config = mkOption {
-        type = types.nullOr hmModule;
+        type =
+          if assertion
+          then types.nullOr hmModule
+          else types.unspecified;
         default = null;
         description = "Home Manager configuration.";
       };
@@ -71,7 +77,20 @@ in
 
   ###### implementation
 
-  config = mkIf (cfg.config != null) {
+  config = mkIf (cfg.config != null) (mkMerge [
+
+    {
+      assertions = [
+        {
+          inherit assertion;
+          message = "You are currently using release-19.09 branch of home-manager, you need "
+            + "to update to the release-20.03 channel.";
+        }
+      ];
+    }
+
+    # hack to determine if cfg.config is a valid home-manager config
+    (mkIf (cfg.config ? home && cfg.config.home ? activationPackage) {
 
       inherit (cfg.config) assertions warnings;
 
@@ -94,5 +113,7 @@ in
 
       environment.packages = mkIf cfg.useUserPackages cfg.config.home.packages;
 
-  };
+    })
+
+  ]);
 }