Implicitly quantifying a kind variable causes GHC 8.6 to panic (coercionKind)
The following program panics on GHC 8.6 and later:
{-# 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 <k_a1js[sk:1]>_N (Nth:0 (Sym {co_a1jI}))))
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler/utils/Outputable.hs:1164:37 in ghc:Outputable
pprPanic, called at compiler/types/Coercion.hs:1887:9 in ghc:Coercion
As noted in the comments, replacing WhySym
with a version that explicitly quantifies z
avoids the panic.
This is a regression from GHC 8.4.3, in which the program simply errored:
$ /opt/ghc/8.4.3/bin/ghc Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:56:38: error:
• Expected kind ‘forall z (y :: z). (a1 :~~: y) ~> *’,
but ‘WhySym a’ has kind ‘forall (y :: z0).
TyFun (a1 :~~: y) * -> *’
• In the type ‘(WhySym a)’
In the expression: (~>:~~:) @j @k @a @b @(WhySym a) @r singEq HRefl
In a case alternative:
SomeSing (singEq :: Sing r)
-> (~>:~~:) @j @k @a @b @(WhySym a) @r singEq HRefl
• Relevant bindings include
singEq :: Sing a2 (bound at Bug.hs:55:23)
eq :: a1 :~~: b (bound at Bug.hs:54:6)
hsym :: (a1 :~~: b) -> b :~~: a1 (bound at Bug.hs:54:1)
|
56 | (~>:~~:) @j @k @a @b @(WhySym a) @r singEq HRefl
| ^^^^^^^^
Trac metadata
Trac field | Value |
---|---|
Version | 8.5 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | high |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |