#13394: PatternSynonyms/OverloadedStrings regression in GHC HEAD -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.2.1 Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple PatternSynonyms | Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- `purescript-0.10.7` fails to build with GHC HEAD at the moment due to an apparent regression in the way pattern synonyms and overloaded strings interact. Here is a simplified example: {{{#!hs {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE PatternSynonyms #-} module Bug where import Data.ByteString newtype ProperName = ProperName { runProperName :: ByteString -- purescript actually uses the Text type, but this works -- just as well for the purposes of illustrating the bug } newtype ModuleName = ModuleName [ProperName] pattern TypeDataSymbol :: ModuleName pattern TypeDataSymbol = ModuleName [ProperName "Type",ProperName "Data", ProperName "Symbol"] }}} Compiling this with GHC 7.10.3 or 8.0.2 works without issue. In GHC HEAD, if you compile this with optimization enabled, it'll trigger a GHC panic: {{{ $ ~/Software/ghc5/inplace/bin/ghc-stage2 -O1 -fforce-recomp Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20170303 for x86_64-unknown-linux): isUnliftedType r_a28T :: TYPE rep_a28S 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/types/Type.hs:1961:10 in ghc:Type }}} The presence of optimization is crucial for reproducing this bug, as compiling it with `-O0` does not trigger the panic. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13394 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler