overlays = [          
  solc.overlays.defaul # <-- ?
];                    

{
  description = "A very basic flake";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
  flake-utils.lib.eachSystem
  [
    "x86_64-linux"
    "x86_64-darwin"
    "aarch64-darwin"
  ] (system:
  let packages = import nixpkgs {
    inherit system;
    # overlays = [ solc-pkgs-overlay ];
  };
  in
  {
    inherit packages;
  });
}

And what's wrong with this flake...

Nix flake check complains:

       error: flake attribute 'packages.x86_64-linux.system' is not a derivation

After updating to 24.05, in order to install a package from 23.11, I added to flake.nix
nixpkgs_old.url = "github:NixOS/nixpkgs/nixos-23.11";
there :

inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.05";
    nixpkgs_unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
    nixpkgs_old.url = "github:NixOS/nixpkgs/nixos-23.11";
  };

and I declare masterpdfeditor like that in my configuration.nix file :

environment.systemPackages = with pkgs; [
    inputs.nixpkgs_old.legacyPackages."${pkgs.system}".masterpdfeditor
  ];
  nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
    "masterpdfeditor"
  ];
  
   nixpkgs.overlays = [ (final: prev: {
     masterpdfeditor = prev.masterpdfeditor.overrideAttrs (old: {
       src = prev.fetchurl {
         url = "https://code-industry.net/public/master-pdf-editor-5.9.10-qt5.x86_64.tar.gz";
         sha256 = "sha256-ilJhUsr96xCbM0XNQxWGAPknKKp1jj+U1+7n1e379Lc=";
       };
     });
   })];
}

Unfortunately, I get the following error :

error: Package ‘masterpdfeditor-5.9.35’ in /nix/store/yg436dyaanpmhi3bf6qizaxa3jymlsx3-source/pkgs/applications/misc/masterpdfeditor/default.nix:40 has an unfree license (‘unfreeRedistributable’), refusing to evaluate.

       a) To temporarily allow unfree packages, you can use an environment variable
          for a single invocation of the nix tools.

            $ export NIXPKGS_ALLOW_UNFREE=1

          Note: When using `nix shell`, `nix build`, `nix develop`, etc with a flake,
                then pass `--impure` in order to allow use of environment variables.

       b) For `nixos-rebuild` you can set
         { nixpkgs.config.allowUnfree = true; }
       in configuration.nix to override this.

       Alternatively you can configure a predicate to allow specific packages:
         { nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
             "masterpdfeditor"
           ];
         }

       c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
         { allowUnfree = true; }
       to ~/.config/nixpkgs/config.nix.

even though,

nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
    "masterpdfeditor"
  ];

is already in configuration.nix file.

In reply to @bryan.bennett:matrix.org

Abílio Costa: I missed your reply here. I am sorry for that! Both nix-direnv and flakeenv take a flake specifier for the flake you want to use: use flake /home/acosta/dev/thirdparty/nix-environments#home-assistant or use flake_env /home/acosta/dev/thirdparty/nix-environments#home-assistant should work, I would think!

I wrote and maintain flakeenv and am a maintainer of nix-direnv, so if you have trouble - please ping me here or (preferably) on the repo for the tool in question :)