#15343: Implicitly quantifying a kind variable causes GHC 8.6 to panic
(coercionKind)
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Reporter: RyanGlScott | Owner: (none)
Type: bug | Status: new
Priority: high | Milestone: 8.6.1
Component: Compiler | Version: 8.5
(Type checker) |
Keywords: TypeInType | 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:
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The following program panics on GHC 8.6 and later:
{{{#!hs
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
module Bug where
import Data.Kind
import Data.Type.Equality
data family Sing :: forall k. k -> Type
data SomeSing :: Type -> Type where
SomeSing :: Sing (a :: k) -> SomeSing k
class SingKind k where
type Demote k :: Type
fromSing :: Sing (a :: k) -> Demote k
toSing :: Demote k -> SomeSing k
data instance Sing (z :: a :~~: b) where
SHRefl :: Sing HRefl
instance SingKind (a :~~: b) where
type Demote (a :~~: b) = a :~~: b
fromSing SHRefl = HRefl
toSing HRefl = SomeSing SHRefl
data TyFun :: Type -> Type -> Type
type a ~> b = TyFun a b -> Type
infixr 0 ~>
type family Apply (f :: k1 ~> k2) (x :: k1) :: k2
(~>:~~:) :: forall (j :: Type) (k :: Type) (a :: j) (b :: k)
(p :: forall (z :: Type) (y :: z). (a :~~: y) ~> Type)
(r :: a :~~: b).
Sing r
-> Apply p HRefl
-> Apply p r
(~>:~~:) SHRefl pHRefl = pHRefl
type family Why (a :: j) (e :: a :~~: (y :: z)) :: Type where
Why a (_ :: a :~~: y) = y :~~: a
data WhySym (a :: j) :: forall (y :: z). (a :~~: y) ~> Type
-- data WhySym (a :: j) :: forall z (y :: z). (a :~~: y) ~> Type
-- The version above does NOT panic
type instance Apply (WhySym a) e = Why a e
hsym :: forall (j :: Type) (k :: Type) (a :: j) (b :: k).
a :~~: b -> b :~~: a
hsym eq = case toSing eq of
SomeSing (singEq :: Sing r) ->
(~>:~~:) @j @k @a @b @(WhySym a) @r singEq HRefl
}}}
{{{
$ /opt/ghc/8.6.1/bin/ghc Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
ghc: panic! (the 'impossible' happened)
(GHC version 8.6.0.20180627 for x86_64-unknown-linux):
coercionKind
Nth:3
(Inst {co_a1jI} (Coh