Breaking bug in 7.8.1

Austin, Simon (and others) As you'll see from https://ghc.haskell.org/trac/ghc/ticket/8978, I succeeded in introducing a new bug into 7.8.1 post RC2, when fixing #8913. And it seems to matter. (Because of the windows seg-fault saga, RC2 was active for a long time, and none of our regression tests showed up this bug.) It looks to me that we have little choice but to push out 7.8.2, more or less immediately, and advise everyone to ignore 7.8.1. Do you agree? Sorry about this. Simon