BangPatterns-related behavior regressions on GHC 8.0

When you compile this code:

{-# LANGUAGE BangPatterns #-}

main :: IO ()
main = let !_ = (undefined :: ()) in print 2

with GHC 8.0.1-rc2 with any level of optimizations, it will print 2 instead of throwing undefined like GHC 7.10.3 does.

Note that changing undefined :: () to something like undefined :: Int# is enough to invoke undefined when compiled, but if you run this code in GHCi:

λ> :set -XMagicHash -XBangPatterns
λ> import GHC.Prim
λ> let !_ = (undefined :: Int#) in print 2

You get something even stranger:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.0.20160204 for x86_64-unknown-linux):
        dsLet: unlifted
  !_ = (undefined :: Int#)
  print @ Integer $dShow_a2ky 2

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

Trac metadata

Trac field	Value
Version	8.0.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
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