Here‘s a thought: doesn’t Generic already have an unused phantom type that's only there “just in case we need to add something in the future”? How about using that to track whether an instance was derived or not, and enforce that with SafeHaskell? Your SafeEq can then use a Generic Derived constraint or whatever.

Ah this sounds promising. You mean that 'x' param in from/to? So to use that, whereever I call to/from I would have to constrain that x to be "Derived".

But do I understand correctly that GHC would have to modified so that its derived instances of Generic force that x param to be Derived?