Thanks for bringing this to my attention; I'll take a look. Excerpts from GHCindex 3a6a4070d2..0fad1da50b 100644's message of 2017-10-27 23:14:13 +0000:

Edward Could you possibly take a look at #14396? It's going to bite us soon, as Alan's Trees That Grow patch lands (see the branch mentioned on the ticket). (In fact, Alan, I don't know how you build that branch ; I can't build it.) Simon