Just for the record, I've opened:
https://ghc.haskell.org/trac/ghc/ticket/13825
to track this.

Cheers,
Michal

On Mon, Jun 12, 2017 at 8:45 PM Michal Terepeta <michal.terepeta@gmail.com> wrote:
Thanks a lot for the replies & links!

I'll try to finish Simon's diff (and probably ask silly questions if I get stuck ;)

Cheers,
Michal