GHC 8.10 regression: Core Lint error when defining instance of CUSK-less class
Here are two files:
-- Lib.hs
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
module Lib where
import Data.Kind
class C (f :: k -> Type) z where
type T (x :: f a) :: f a
-- App.hs
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
module App where
import Data.Kind
import Data.Proxy
import Lib
type family F (x :: Proxy a) :: Proxy a
instance C Proxy z where
type T x = F x
Compiling this with GHC 8.8.1 passes Core Lint:
$ /opt/ghc/8.8.1/bin/ghc -c Lib.hs && /opt/ghc/8.8.1/bin/ghc -c App.hs -dcore-lint
But it does not pass Core Lint on GHC 8.10.1-alpha1:
$ /opt/ghc/8.10.1/bin/ghc -c Lib.hs && /opt/ghc/8.10.1/bin/ghc -c App.hs -dcore-lint
ghc: panic! (the 'impossible' happened)
(GHC version 8.10.0.20191122:
Core Lint error
<no location info>: warning:
The variable @ k1_aiz is out of scope
Substitution: [TCvSubst
In scope: InScope {}
Type env: []
Co env: []]
T
[TCvSubst
In scope: InScope {k1_aiz a_awk x_awl}
Type env: [aig :-> x_awl, awg :-> a_awk]
Co env: []]
[a_awk, x_awl]
[]
[k1_aiz, Proxy, a_awk, x_awl]
F x_awl
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler/utils/Outputable.hs:1179:37 in ghc:Outputable
pprPanic, called at compiler/typecheck/FamInst.hs:184:31 in ghc:FamInst
Another curious thing is that if you load Lib.hs
into GHCi, you'll observe something strange about the kind of T
:
$ /opt/ghc/8.10.1/bin/ghci Lib.hs
GHCi, version 8.10.0.20191122: https://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Lib ( Lib.hs, interpreted )
Ok, one module loaded.
λ> :i T
type C :: forall k k1. (k1 -> *) -> k -> Constraint
class C f z where
type T :: forall k2 (f :: k1 -> *) (a :: k2). f a -> f a
type family T x
-- Defined at Lib.hs:11:3
Note that T
has kind forall k2 (f :: k1 -> *) (a :: k2). f a -> f a
, which does not appear to be well kinded. It's unclear to me if this is the cause behind the Core Lint error or not.