The fromJust error is a bug, of course, however, the underlying
problem is a bit more difficult:

Haddock doesn't generate any code, it only typechecks. If the code
uses Template Haskell, however, the typechecker will have to run
Haskell code and potentially this code will have to come from a module
of the same package. If the code indeed comes from the same package,
fixing the fromJust error will just lead to GHCi linker error, since
Haddock didn't generate any code for these.

Thanks for enlightening us, Thomas.

Here are a couple of solutions and non-solutions:

[...]

Any other?

What about the eventual (maybe never?) solution of collecting comments and types for Haddock after splicing in code? This is more long term, perhaps, but in the end ideal if it can be made to work.

Regards,
Sean