GHC HEAD-only panic (zonkTcTyVarToTyVar)
The following program typechecks on GHC 8.0.1 through 8.6.1:
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
module Bug where
import Data.Kind
type a ~> b = a -> b -> Type
type family Apply (f :: k1 ~> k2) (x :: k1) :: k2
data family Sing (a :: k)
data Flarble (a :: Type) where
MkFlarble :: Flarble Bool
data instance Sing (z :: Flarble a) where
SMkFlarble :: Sing MkFlarble
elimFlarble :: forall a
(p :: forall x. Flarble x ~> Type)
(f :: Flarble a).
Sing f
-> Apply p MkFlarble
-> Apply p f
elimFlarble s@SMkFlarble pMkFlarble =
case s of
(_ :: Sing (MkFlarble :: Flarble probablyABadIdea)) ->
pMkFlarble
However, it panics on GHC HEAD:
$ ~/Software/ghc/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
ghc-stage2: panic! (the 'impossible' happened)
(GHC version 8.7.20181015 for x86_64-unknown-linux):
zonkTcTyVarToTyVar
probablyABadIdea_aWn[tau:2]
Bool
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
pprPanic, called at compiler/typecheck/TcMType.hs:1627:34 in ghc:TcMType
If I replace probablyABadIdea
with Bool
, then it typechecks again.
Trac metadata
Trac field | Value |
---|---|
Version | 8.5 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | highest |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |