[GHC] #14873: GHC HEAD regression (piResultTy)

[GHC]

#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:
          Component:  Compiler       |           Version:  8.5
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 (Originally noticed [https://travis-
 ci.org/goldfirere/singletons/jobs/347945148#L1179 here].)

 The following program typechecks on GHC 8.2.2 on GHC 8.4.1, but panics on
 GHC HEAD:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Bug where

 import Data.Kind (Type)

 data family Sing (a :: k)

 newtype instance Sing (f :: k1 ~> k2) =
   SLambda { applySing :: forall t. Sing t -> Sing (Apply f t) }

 data TyFun :: Type -> Type -> Type
 type a ~> b = TyFun a b -> Type
 infixr 0 ~>
 type family Apply (f :: k1 ~> k2) (x :: k1) :: k2

 class SingI (a :: k) where
   sing :: Sing a

 data ColSym1 :: f a -> a ~> Bool
 type instance Apply (ColSym1 x) y = Col x y

 class PColumn (f :: Type -> Type) where
   type Col (x :: f a) (y :: a) :: Bool

 class SColumn (f :: Type -> Type) where
   sCol :: forall (x :: f a) (y :: a).
     Sing x -> Sing y -> Sing (Col x y :: Bool)

 instance (SColumn f, SingI x) => SingI (ColSym1 (x :: f a) :: a ~> Bool)
 where
   sing = SLambda (sCol (sing @_ @x))
 }}}

 {{{
 $ /opt/ghc/head/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.5.20180201 for x86_64-unknown-linux):
         piResultTy
   k_aZU[tau:1]
   (a_aW8[sk:1] |> <*>_N)
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler/types/Type.hs:947:35 in ghc:Type
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873>
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