#14038: TypeApplications regression in GHC HEAD: ‘p0’ is untouchable inside the constraints: () -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.3 checker) | Keywords: Resolution: | TypeApplications Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13877 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Richard, in a `-DDEBUG` compiler this falls over with {{{ Following filled tyvar a_a225[tau:2] = a_a220[sk:2] Unfilled tyvar a_a220[sk:2] flatten/appty (p0_a226[tau:2] |> T14038.D:R:Funk1:->k2[0] <k1>_N <k2>_N) '[] p0_a226[tau:2] '[] nominal nominal flatten } p0_a226[tau:2] '[] New coercion hole: a23h Emitting new coercion holeghc-stage1: panic! (the 'impossible' happened) (GHC version 8.3.20170726 for x86_64-unknown-linux): piResultTy Fun [a_a220[sk:2]] (':->) * '[] Call stack: ?callStack, called at compiler/utils/Util.hs:1394:50 in ghc:Util 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/types/Type.hs:949:35 in ghc:Type Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} I think what is happening is that we start with `(p0_a226 |> co) '[]`, where `p0_a226 :: Fun [a_a220[sk:2]] (':->) *`, and `co` casts it to a function type. But then flattening throws away the cats, apparently, and we try to form `(mkAppTy p0_a226 '[])`, which is simply ill-kinded. I have (again) lost track of what is supposed to happen here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14038#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler