Last time I checked GHC's AST types had some fields which you weren't
supposed to look at. Depending on what stage you're in those may be
different fields. They are initialised to error thunks, hence
automatically derived Show instances wouldn't work out of the box as
they would force those error thunks.

As I said, it's been a while since I looked at that closely, so maybe
it was removed when the Typeable instances were moved into the GHC
codebase?

I noticed that some of those are still around. I'm mainly looking at the AST post type-checking.