`nix flake check` fails for overlays that check any attrs during construction of the attrset

[mitchmindtree]

I'm unsure where to best open this, but it looks like this might be due to the schema determinate nix uses for nix flake check by default?

I started digging into this when running into the following CI errors during a nix flake check invocation:

error (ignored): attribute 'lib' missing
error:
       … while calling the 'isAttrs' builtin
         at «builtin-flake-schemas»/flake.nix:258:19:
          257|                 else
          258|                   builtins.isAttrs (overlay { } { });
             |                   ^
          259|             }) output

       … in the right operand of the update (//) operator
         at /home/runner/work/mindtrix/mindtrix/overlays/default.nix:30:3:
           29|   }
           30|   // (prev.lib.optionalAttrs prev.stdenv.isLinux {
             |   ^
           31|     # Disko scripts for mindlane partitioning and mounting.

       error: attribute 'lib' missing
       at /home/runner/work/mindtrix/mindtrix/overlays/default.nix:30:7:
           29|   }
           30|   // (prev.lib.optionalAttrs prev.stdenv.isLinux {
             |       ^
           31|     # Disko scripts for mindlane partitioning and mounting.
Error: Process completed with exit code 1.

It seems like an empty attrset is being provided to the prev argument of the function to test if the overlay is correctly producing an attrset. This causes a failure in the case that any of the attrs are used in the construction of the attrset itself (e.g. checking prev.stdenv.hostPlatform.isLinux).

It seems reasonable for an overlay to assume some basic content of the nixpkgs set might exist like stdenv no? The current nix flake check behaviour implies this is not the case.

Temporary Workaround

Set determinate: false in the nix-installer-action to use the normal nix behaviour.

[mirkolenz]

Ran into this too with multiple projects, for instance https://github.com/mirkolenz/makejinja/actions/runs/22835652823/job/66231501613?pr=302

Would be great if you could address this, so far this is the only bug I encountered with flake schemas, apart from that it's a huge improvement!

[rvolosatovs]

Looks like I've ran into this as well:

• https://github.com/rvolosatovs/nixify/actions/runs/22950641462/job/66614135840?pr=490
• https://github.com/rvolosatovs/nixify/blob/bcc5e78d12c615529c0f40fb40360d84980d25e4/lib/rust/mkAttrs.nix#L748-L749

Caused by: https://github.com/DeterminateSystems/nix-src/pull/217/changes#diff-3e1262572f2f16dc7002cec3cf845bd48c0f1e4daa3cda882720a8ad3d16af82R257