nixos/services.xserver: Fix legacy options for default wm without dm

We switched to unified default session option services.xserver.displayManager.defaultSession
and included fallback path for the legacy options. Unfortunately when only
services.xserver.windowManager.default is set and not services.xserver.desktopManager.default,
it got incorrectly converted to the new option.

This should fix that.

Closes: https://github.com/NixOS/nixpkgs/issues/76684

1 files changed, 4 insertions, 2 deletions

diff --git a/nixos/modules/services/x11/display-managers/default.nix b/nixos/modules/services/x11/display-managers/default.nix
index 1efd0739376..821886e5fda 100644
--- a/nixos/modules/services/x11/display-managers/default.nix
+++ b/nixos/modules/services/x11/display-managers/default.nix
@@ -141,9 +141,11 @@ let
     '';
 
   dmDefault = cfg.desktopManager.default;
+  # fallback default for cases when only default wm is set
+  dmFallbackDefault = if dmDefault != null then dmDefault else "none";
   wmDefault = cfg.windowManager.default;
 
-  defaultSessionFromLegacyOptions = concatStringsSep "+" (filter (s: s != null) ([ dmDefault ] ++ optional (wmDefault != "none") wmDefault));
+  defaultSessionFromLegacyOptions = dmFallbackDefault + optionalString (wmDefault != null && wmDefault != "none") "+${wmDefault}";
 
 in
 
@@ -358,7 +360,7 @@ in
             { c = wmDefault; t = "- services.xserver.windowManager.default"; }
             ]))}
           Please use
-            services.xserver.displayManager.defaultSession = "${concatStringsSep "+" (filter (s: s != null) [ dmDefault wmDefault ])}";
+            services.xserver.displayManager.defaultSession = "${defaultSessionFromLegacyOptions}";
           instead.
         ''
       ];