#11572: BangPatterns-related behavior regressions on GHC 8.0 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: highest | Milestone: 8.0.1 Component: Compiler | Version: 8.0.1-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When you compile this code: {{{#!hs {-# 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 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11572 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler