Enabling both module cmt.nix and plasma5.nix throws a failed assertion and build aborts

[kcalvinalvin]

Describe the bug
In configuration.nix, if you enable both xserver.services.desktopManager.plasma5 and xserver.services.cmt, assertion fails.

assertion = !config.services.xserver.libinput.enable;

Failed assertions:

• cmt and libinput are incompatible, you cannot enable both (in services.xserver).

To Reproduce
Steps to reproduce the behavior:

1. Enable both desktopManager.plasma5.enable and cmt.enable

Expected behavior
Failed assertion should happen

Additional context
Build with xserver.services.desktopManager.xfce works fine.

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 4.19.75, NixOS, 20.03.git.6262e83 (Markhor)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information: kcalvinalvin

a list of nixpkgs attributes affected by the problem
attribute: xserver.services.desktopManager.plasma5, xserver.services.cmt

a list of nixos modules affected by the problem
module: plasma5.nix, cmt.nix

[kcalvinalvin]

nixpkgs/nixos/modules/services/x11/desktop-managers/plasma5.nix

Line 214 in e490494

services.xserver.libinput.enable = mkDefault true;

Found the culprit. Can we disable by default so that users can choose? The default generated configuration.nix has the option by default so I don't see the reason why it needs to also be defaulted in plasma5.nix