Assert that fish configuration is enabled if any user has fish as their shell.

              users.groups.${user.name} = {};

            '';

          }

          {

            assertion = (user.shell == pkgs.fish) -> (config.programs.fish.enable == true);

            message = ''

              users.users.${user.name}.shell is set to fish, but

              programs.fish.enable is not true. This will cause the fish shell

              to lack the basic nix directories in its PATH and might make

              logging in as that user impossible. You can fix it with:

              programs.fish.enable = true;

            '';

          }

        ]

    ));