Repackage as an 'extension'?

[cmichelenstrofer]

Could this be repackaged as an extension, so that it is automatically added if both Unitful and ChainRules are used?

[SBuercklin]

Absolutely, in my opinion this is a perfect use case for package extensions.

I don't have time to handle that right now, but I support migration into the core package and conditional loading from there. If you (or someone) ports it, let me know and I can put a note up on this repo. Might also be good to throw a deprecation warning if it's completely superfluous