Re: WARNING pragmas in `Prelude.undefined` and `Prelude.error`

I assume a TotalHaskell pragma was proposed in the past. Would this help?

Yes, in fact I think this is even better. Does "total" refer to exhaustive pattern matching and absence of (possible) exceptions? We might want to have such a pragma on a function-by-function basis as well as whole-module. My company has committed to letting me work on GHC a couple of days each month, so I'd be up to work on this, although I'd need someone to hold my hand as I haven't done this before.