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
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.0.1

changed weight to 10

added Tbug Trac import labels

Another strange facet of the GHCi portion of the bug: the behavior is different depending on whether a wildcard or a named type variable is used. Here's another bizarre error you can get in GHCi (but not compiled code):

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

<interactive>:3:5: error:
    • You can't mix polymorphic and unlifted bindings
        !a = (undefined :: Int#)
        Probable fix: add a type signature
    • In the expression: let !a = (undefined :: Int#) in print 2
      In an equation for ‘it’:
          it = let !a = (undefined :: Int#) in print 2

This works without issue in GHCi 7.10.3.

mentioned in issue #11601 (closed)

mentioned in commit 01449eb5

Trac metadata

Trac field	Value
Test case	- → deSugar/should_run/T11572

added 1 deleted label

closed

Merged in d6ea90a2.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

added Phighest label