As mentioned in https://mail.haskell.org/pipermail/ghc-devs/2016-February/011455.html, the following tests currently hit an assert when the compiler is built with -DDEBUG:

   patsyn/should_compile  MoreEx [exit code non-0] (normal)
   patsyn/should_compile  T11224b [exit code non-0] (normal)
   polykinds              MonoidsTF [exit code non-0] (normal)
   polykinds              T11480b [exit code non-0] (normal)
   polykinds              T11523 [exit code non-0] (normal)

Compile failed (status 256) errors were:
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.1.20160218 for x86_64-unknown-linux):
ASSERT failed!
  CallStack (from HasCallStack):
  assertPprPanic, called at compiler/types/TyCoRep.hs:1932:56 in ghc:TyCoRep
  checkValidSubst, called at compiler/types/TyCoRep.hs:1991:17 in
ghc:TyCoRep
  substTys, called at compiler/types/TyCoRep.hs:2012:14 in ghc:TyCoRep
  substTheta, called at compiler/typecheck/TcPatSyn.hs:255:20 in
ghc:TcPatSyn
  in_scope InScope {d_ap0 c_apv}
  tenv [ap1 :-> c_apv[tau:5]]
  tenvFVs [aps :-> t_aps[tau:1], apv :-> c_apv[tau:5]]
  cenv []
  cenvFVs []
  tys []
  cos []

D1951 contained a stopgap patch, but since it wasn't accepted, I'll just mark the tests as expect_broken for this ticket, so that other developers aren't bothered by this issue.

The offending commit is b5292557.