error: attribute 'class' missing when mixing module system versions

[roberth]

Describe the bug

The following error occurs since when mixing older versions of the module system with #197547.
It's recommended not to mix module system versions, but I have a fix in mind.

Steps To Reproduce

Steps to reproduce the behavior:

1. Mix module system versions.
2. Evaluate
3. See the error, with the significant bits:

       error: attribute 'class' missing

       at /nix/store/<hash>-source/lib/types.nix:778:41:

          777|             class =
          778|               if lhs.class == null then rhs.class
             |                                         ^
          779|               else if rhs.class == null then lhs.class

Expected behavior

At least adjacent versions of the module system interoperate.

Screenshots

Additional context

• module system: Improve error messages around faulty imports #197547

Notify maintainers

@roberth

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
output here