[GHC] #14246: Probably AllowAmbiguousTypes + UndecidableInstances + TypeInType

[GHC]

#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>
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