This change causes an error:

building the system configuration...
trace: Default graphical session, 'xmonad', not found.
Valid names for 'services.xserver.displayManager.defaultSession' are:
  

trace: Default graphical session, 'xmonad', not found.
Valid names for 'services.xserver.displayManager.defaultSession' are:
  

error: The option value `services.xserver.displayManager.defaultSession' in `/etc/nixos/configuration.nix' is not of type `session name'.
(use '--show-trace' to show detailed location information)

I've tried using both "none+xmonad" and "xmonad" as the defaultSession:

  services.xserver = {
     enable = true;
     windowManager.xmonad = {
       enable = true;   
     };

    displayManager.defaultSession = "xmonad";
    displayManager.lightdm = {
	enable = true;
     };
  };

I tested reverting the change and setting defaultSession to "none+xmonad" and it worked.

I reverted it. I'm wondering is it that none gets imported default so a none desktop manager is somehow always an option 😕 (but it isn't a select-able?)

In that case, I should really add a comment here because the files look like lost fragments that are very often in nixpkgs.

My understanding is that the selectable sessions are generated by combining every enabled window manager with every enabled desktop manager; the Cartesian product of desktop and window managers. But with the caveat that the combination of "none+none" is ignored, which is why there's no "none" session available.

In my case this (bisected) caused a different error: after a successful switch to the new configuration, sddm does not list the available sessions (in the lower left corner) and does not launch any session when I login (one of my users uses stock plasma5, another custom .xsession). My configuration is:

services.xserver.displayManager.sddm.enable = true;
services.xserver.desktopManager.plasma5.enable = true;

In my case this change made it impossible to login with lightdm because /nix/store/XXXX-desktops/share/xsessions was empty:

[+68.72s] DEBUG: Seat seat0: Failed to find session configuration xsession
[+68.72s] DEBUG: Seat seat0: Can't find session 'xsession'

I just stumbled upon this after my system failed to evaluate due to sessionPackages being empty. Maybe it should be documented in nixos/modules/services/x11/desktop-managers/none.nix that the crossList logic requires it to be present?

I will look into making it appear more idiomatic. But it's been reverted already.