#15002: Panic: collectNBinders -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.4.2 Component: Profiling | Version: 8.4.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * priority: normal => highest * version: 8.4.2-rc1 => 8.4.1 * component: Compiler => Profiling Comment: Thanks for the bug report. Note that one can also reproduce this with GHC 8.4.1 as well. Here is a minimal example: {{{#!hs module Bug where import Control.Concurrent.MVar (MVar, modifyMVar_, putMVar) import Data.Foldable (for_) broadcastThen :: Either [MVar a] a -> MVar (Either [MVar a] a) -> a -> IO () broadcastThen finalState mv x = modifyMVar_ mv $ \mx -> do case mx of Left ls -> do for_ ls (`putMVar` x) return finalState Right _ -> return finalState }}} {{{ $ ~/Software/ghc-8.4.1/bin/ghc -fprof-auto -prof -fforce-recomp -O1 Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.4.1 for x86_64-unknown-linux): collectNBinders 1 Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable pprPanic, called at compiler/coreSyn/CoreSyn.hs:2189:39 in ghc:CoreSyn }}} Note that this bug goes away if one removes the `-O1` or `-fprof-auto` flags. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15002#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler