#15778: GHC HEAD-only panic (zonkTcTyVarToTyVar) -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.8.1 Component: Compiler (Type | Version: 8.7 checker) | Resolution: | Keywords: TypeInType 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: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): The following code does //not// compile on earlier versions of GHC, but it is the simplest way to trigger the panic that I've discovered: {{{#!hs {-# LANGUAGE PolyKinds, ScopedTypeVariables #-} foo (_ :: (a :: probablyABadIdea)) = () }}} {{{ $ /opt/ghc/head/bin/ghc Bug.hs [1 of 1] Compiling Main ( Bug.hs, Bug.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.7.20181015 for x86_64-unknown-linux): zonkTcTyVarToTyVar probablyABadIdea_arQ[tau:1] TYPE t_arX[tau:1] Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable pprPanic, called at compiler/typecheck/TcMType.hs:1627:34 in ghc:TcMType }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15778#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler