#13458: Panic with unsafeCoerce and -dcore-lint -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.1 (Type checker) | 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: -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE MagicHash, TypeInType, ScopedTypeVariables #-} module UIOST where import GHC.Exts import Data.Kind import Unsafe.Coerce unsafeCoerce' :: forall (r :: RuntimeRep) (a :: TYPE r) (b :: TYPE r). a -> b unsafeCoerce' = unsafeCoerce unsafeCoerce }}} Compiling this with optimization and `-dcore-lint` produces a panic: {{{ ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20170308 for x86_64-unknown-linux): runtimeRepPrimRep typePrimRep (a_aW7 :: TYPE r_aW6) r_aW6 Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1191:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1195:37 in ghc:Outputable pprPanic, called at compiler/simplStg/RepType.hs:360:5 in ghc:RepType }}} Using `unsafeCoerce# unsafeCoerce` produces the panic without optimization. I have not been able to reproduce the problem without `-dcore-lint`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13458 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler