I got the following error compiling simple source file Bad.hs. The bug is likely related to TH as the crash vanishes without the TH line.

$ stack ghc -- -c Bad.hs
Run from outside a project, using implicit global project config
Using resolver: lts-7.5 from implicit global project's config file: /home/jchia/.stack/global-project/stack.yaml
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-unknown-linux):
        compiler/typecheck/TcExpr.hs:798:15-35: Irrefutable pattern failed for pattern sel_id : _


Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug