Installing different versions of node and npm deviates from nix behavior

[dit7ya]

Describe the bug

With the following flake.nix

{
  description = "Node dev";
  inputs.flake-utils.url = "github:numtide/flake-utils";
  inputs.devshell.url = "github:numtide/devshell";

  outputs = { self, nixpkgs, flake-utils, devshell }:

    flake-utils.lib.eachDefaultSystem (system: {
      devShell = let
        pkgs = import nixpkgs {
          inherit system;

          overlays = [ devshell.overlay ];
        };
      in pkgs.devshell.mkShell {
        imports = [ (pkgs.devshell.importTOML ./devshell.toml) ];
        devshell.packages = [ pkgs.nodejs-14_x pkgs.nodePackages.npm ];

      };
    });

nix develop results into error

error: builder for '/nix/store/yxj3h7c72xygkb9i88w7kqz7nykqf647-devshell-dir.drv' failed with exit code 25;
       last 1 log lines:
       > error: collision between `/nix/store/x22pf8sm4ym58s2zqhjzqdz649a6yq44-npm-8.3.2/lib/node_modules/npm/README.md' and `/nix/store/p88qhhnrwsi5rxfsz3cgvgs80lkgplp1-nodejs-14.18.3/lib/node_modules/npm/README.md'
       For full logs, run 'nix log /nix/store/yxj3h7c72xygkb9i88w7kqz7nykqf647-devshell-dir.drv'.
error: 1 dependencies of derivation '/nix/store/vvswra82ipmcbm3vk4fndvxj98rqcp4m-devshell-env.drv' failed to build

Whereas in regular flake.nix both the packages are installed and gives you a shell with node 14 and npm 8.

I can understand if this is a feature rather than a bug. In that case please let me know what is a workaround.

[NickHu]

You can use lib.lowPrio and lib.hiPrio to de/prioritise packages.