error: expected a string but got a thunk when importing a flake input

[yajo]

Is your feature request related to a problem? Please describe.
While developing https://gitlab.com/moduon/devsecops/precommix, I wanted a way to force the flake to get a specific input version depending on how the Copier template is updated.

Describe the solution you'd like
I thought it would be pretty simple to add a .precommix.nix file, updated by the Copier template, and then just add to the flake:

inputs.precommix.url = import ./.precommix.nix;

However, if you put such import in a flake input, you get this error:

error: expected a string but got a thunk at /nix/store/jymd3dfmwvv0k7f2nbrqgf4pp9w6ax4s-source/flake.nix:10:3

Describe alternatives you've considered
Not sure... using builtins.getFlake?

Additional context
Precommix combines the best parts of pre-commit and nix.

To make sure we get full reproducibility, I want to share the same commit between the .pre-commit-config.yaml file and flake.lock when updating the template.

To manage template I use https://copier.readthedocs.io/ because it supports smart updates.

[dasJ]

Seems like a duplicate of #4945

[yajo]

Indeed.