overlay-conditional-override.nix

let 
  overlay = self: super: rec {
    my-hello = super.callPackage ./pkgs/hello/default.nix {};
    hello = if super.hello.version > my-hello.version then super.hello else my-hello;
  };
  unstable = import <nixos-unstable> {
    overlays = [ overlay ];
    config = { };
  };
in 
{
  environment.systemPackages = [
    unstable.hello
  ];
}