#16191: GHC Panic deriving unbox vector (StrictData/unbox-strict-fields interaction) -------------------------------------+------------------------------------- Reporter: dbeacham | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Keywords: | Operating System: Linux Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The attached program fails to compile with the following error: {{{ ghc: panic! (the 'impossible' happened) (GHC version 8.6.3 for x86_64-unknown-linux): mkNewTyConRhs MV_V [Int#, MVector s_a7WC[sk:1] a_a7WA[sk:1], MVector s_a7WC[sk:1] a_a7WA[sk:1]] Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable pprPanic, called at compiler/iface/BuildTyCl.hs:65:27 in ghc:BuildTyCl }}} Removing either of the -funbox-strict-fields or StrictData pragma allow the code to compile correctly, but you do need to compile with -O to get the incorrect behaviour -ghci is fine loading the module. Although I wouldn't expect them to interact here as I don't get the same behaviour if I explicitly write out what would be implied by the flag and pragma: {{{ data V a = V {-# UNPACK #-} !a {-# UNPACK #-} !a deriving (Show) }}} just warnings about the unused UNPACKs. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16191 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler