nix flake check fails with overlays because arguments are not properly named

[mitchellh]

Describe the bug

If you specify an overlay attribute when using flake-utils.lib.eachSystem, nix flake check will fail because the final overlay attribute has invalid argument names.

To Reproduce

Steps to reproduce the behavior:

1. Create a flake with an overlay
2. Run nix flake check

My flake is roughly this:

{
  outputs = { self, nixpkgs, flake-utils, ... }@inputs:
    flake-utils.lib.eachSystem ["aarch64-linux" "x86_64-linux"] (system: {
        # ... other stuff

        overlay = final: prev: {
          foo = packages.foo;
        };
      }
    );
}

Error:

$ nix flake check --show-trace
error: overlay does not take an argument named 'final'

       … while checking the overlay 'overlay'

       at /nix/store/kfpqwqdzm3rf153hr63hp4v81xbm5bx4-source/default.nix:75:15:

           74|             {
           75|               ${key} = (attrs.${key} or { }) // (if key == "hydraJobs"
             |               ^
           76|               then builtins.mapAttrs (name: value: (attrs.hydraJobs.${name} or { }) // { ${system} = value; }) ret.hydraJobs

       … while checking flake output 'overlay'

       at /nix/store/kfpqwqdzm3rf153hr63hp4v81xbm5bx4-source/default.nix:75:15:

           74|             {
           75|               ${key} = (attrs.${key} or { }) // (if key == "hydraJobs"
             |               ^
           76|               then builtins.mapAttrs (name: value: (attrs.hydraJobs.${name} or { }) // { ${system} = value; }) ret.hydraJobs

Expected behavior

nix flake check should pass.

Additional Context

$ nix --version
nix (Nix) 2.5pre20211007_844dd90

[zimbatm]

eachSystem is just for the attributes that are organized by <attr>.<system>. You can merge it with the other ones like that:

{
  outputs = { self, nixpkgs, flake-utils, ... }@inputs:
    (flake-utils.lib.eachSystem ["aarch64-linux" "x86_64-linux"] (system: {
        # ... other stuff
      }
    )) // {
      overlay = final: prev: {
        foo = packages.foo;
      };
    };
}

[mitchellh]

Got it thanks!