Hello Maxime!

> I've seen this before (for something non-libtree), IIUC you can only
> run '--check' after it has been built (or substituted) for the first
> time.

Thanks for pointing that out, I overlooked this little detail somehow.

When I removed '--check' from Guix options it built libtree without
errors with your version of the patch.

Please find the updated patch attached.

- Artyom