#15588: Panic when abusing kind inference -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Keywords: TypeInType | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When I say {{{#!hs {-# LANGUAGE ScopedTypeVariables, TypeInType, TypeOperators, TypeFamilies, AllowAmbiguousTypes #-} import Data.Proxy import Data.Type.Equality import Data.Type.Bool import Data.Kind data SameKind :: forall k. k -> k -> Type type family IfK (e :: Proxy (j :: Bool)) (f :: m) (g :: n) :: If j m n where IfK (_ :: Proxy True) f _ = f IfK (_ :: Proxy False) _ g = g y :: forall ck (c :: ck). ck :~: Proxy True -> () y Refl = let x :: forall a b (d :: a). SameKind (IfK c b d) d x = undefined in () }}} HEAD says {{{ ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.7.20180827 for x86_64-apple-darwin): ASSERT failed! Bad coercion hole co_a3iZ: If j_a3j0[tau:2] m_a3j1[tau:2] a_a3gV[sk:3] a_a3gV[sk:3] nominal If j_a3j0[tau:2] m_a3j1[tau:2] a_a3jj[sk:3] ~# a_a3jj[sk:3] Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable pprPanic, called at compiler/utils/Outputable.hs:1219:5 in ghc:Outputable assertPprPanic, called at compiler/typecheck/TcMType.hs:316:25 in ghc:TcMType Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} It's as yet unclear whether the program should be accepted. My best guess is that it should, but that (even with this panic fixed) GHC isn't up to the task. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15588 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler