#13264: GHC panic with (->) generalization branch while compiling lens -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by bgamari: @@ -1,1 +1,1 @@ - While testing characterizing the performance impact of the Typeable + While characterizing the performance impact of the Typeable New description: While characterizing the performance impact of the Typeable branch (`wip/ttypeable`) against Hackage packages I have found that `lens` manages to break the `(->)` kind-generalization patch. Specifically, `TcCanonical.can_eq_nc` induces a panic by `tcRepSplitTyApp_maybe` during compilation of `Control.Lens.Traversal.holesOf`, {{{ ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20170207 for x86_64-unknown-linux): tcRepSplitTyConApp_maybe ([] |> <*>_N ->_N Sym {alzj}) a_alzd[tau:5] c_alzb[tau:5] Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1188:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1192:37 in ghc:Outputable pprPanic, called at compiler/typecheck/TcType.hs:1456:5 in ghc:TcType tcRepSplitTyConApp_maybe, called at compiler/typecheck/TcCanonical.hs:617:25 in ghc:TcCanonical }}} The last thing emitted by tc-trace is, {{{ can_eq_nc False [WD] hole{alyP} {0}:: (p_alyn[tau:5] :: TYPE p_alym[tau:5]) GHC.Prim.~# (cat_alyK[tau:6] b_alyM[tau:6] c_alyN[tau:6] :: *) nominal equality ([] |> <*>_N ->_N Sym {alzj}) a_alzd[tau:5] -> c_alzb[tau:5] p_alyn[tau:5] cat_alyK[tau:6] b_alyM[tau:6] c_alyN[tau:6] cat_alyK[tau:6] b_alyM[tau:6] c_alyN[tau:6] }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13264#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler