#13984: Panic when using a TH splice in a do bind pattern -------------------------------------+------------------------------------- Reporter: mbieleck | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc3 Keywords: panic | Operating System: Unknown/Multiple template-haskell | Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When compiling this file with HEAD (f656fba19d0cefe05643ddea35d080ea332a6584): {{{#!hs {-# LANGUAGE TemplateHaskell #-} module Panic where import Language.Haskell.TH expr :: IO Exp expr = runQ $ do name <- newName "foo" [| do $(varP name) <- pure (); pure () |] }}} I get this message: {{{ [1 of 1] Compiling Panic ( Panic.hs, Panic.o ) ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.3.20170713 for x86_64-unknown-linux): isIrrefutableHsPat: $(varP name) Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable pprPanic, called at compiler/hsSyn/HsPat.hs:643:15 in ghc:HsPat Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} It compiles normally with GHC 8.2.1-rc2 (20170507). The bug exists in rc3 (20170704). I am unable to perform a more precise bisect. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13984 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler