#14246: Probably AllowAmbiguousTypes + UndecidableInstances + TypeInType -------------------------------------+------------------------------------- Reporter: Toricon | Owner: (none) Type: bug | Status: infoneeded Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: x86_64 Type of failure: Compile-time | (amd64) crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: #13271 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I was able to reverse engineer this. Here is a program which panics on GHC 8.0.2: {{{#!hs {-# LANGUAGE RankNTypes, GADTs, TypeOperators, PolyKinds, DataKinds, TypeFamilies, AllowAmbiguousTypes, UndecidableInstances, TypeInType #-} module MCS.Function where -- import MCS.Core --import MCS.Value import Data.Kind -- necessary for * data Nat = Z | S Nat data Vect :: Nat -> Type -> Type where Nil :: Vect Z a Cons :: a -> Vect n a -> Vect (S n) a data Label a = Label a data L type family KLN (n :: k) :: Nat where KLN (f :: v -> k) = S (KLN (forall t. f t)) KLN (f :: *) = Z type family Reveal (n :: k) (l :: Vect (KLN n) L) :: * where Reveal (f :: v -> k) (Cons (Label (t :: v)) l) = Reveal (f t) l Reveal (a :: *) Nil = a }}} {{{ $ /opt/ghc/8.0.2/bin/ghc Bug.hs [1 of 1] Compiling MCS.Function ( Bug.hs, Bug.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-linux): isInjectiveTyCon sees a TcTyCon KLN }}} However, this does //not// panic on GHC 8.2.2 and later, so I believe that this has a symptom in common with #13271. It's different enough that it warrants its own test case, I believe, so I'll commit that shortly. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14246#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler