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The following program gives a Core Lint error on GHC 8.2.2 and later:

{-# LANGUAGE EmptyCase #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE UndecidableInstances #-}
module Bug where

import Data.Kind
import Data.Void

data family Sing (a :: k)

data V1 :: Type -> Type
data instance Sing (z :: V1 p)

class Generic a where
  type Rep a :: Type -> Type
  to :: Rep a x -> a

class PGeneric a where
  type To (z :: Rep a x) :: a

class SGeneric a where
  sTo :: forall x (r :: Rep a x). Sing r -> Sing (To r :: a)

-----

instance Generic Void where
  type Rep Void = V1
  to x = case x of {}

type family EmptyCase (a :: j) :: k where

instance PGeneric Void where
  type To x = EmptyCase x

instance SGeneric Void where
  sTo x = case x of

$ /opt/ghc/8.4.3/bin/ghc Bug.hs -dcore-lint -fforce-recomp       
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
*** Core Lint errors : in result of Simplifier ***
Bug.hs:38:7: warning:
    [in body of lambda with binder x_aZ8 :: Sing r_a12q]
    Kind application error in
      coercion ‘(Sing (D:R:RepVoid[0] <x_a12p>_N) <r_a12q>_N)_R’
      Function kind = forall k. k -> *
      Arg kinds = [(V1 x_a12p, *), (r_a12q, Rep Void x_a12p)]
    Fun: V1 x_a12p
         (r_a12q, Rep Void x_a12p)
Bug.hs:38:7: warning:
    [in body of lambda with binder x_aZ8 :: Sing r_a12q]
    Kind application error in
      coercion ‘D:R:SingV1z0[0] <x_a12p>_N <r_a12q>_N’
      Function kind = V1 x_a12p -> *
      Arg kinds = [(r_a12q, Rep Void x_a12p)]
    Fun: V1 x_a12p
         (r_a12q, Rep Void x_a12p)
Bug.hs:38:7: warning:
    [in body of lambda with binder x_aZ8 :: Sing r_a12q]
    Kind application error in
      coercion ‘D:R:SingV1z0[0] <x_a12p>_N <r_a12q>_N’
      Function kind = V1 x_a12p -> *
      Arg kinds = [(r_a12q, Rep Void x_a12p)]
    Fun: V1 x_a12p
         (r_a12q, Rep Void x_a12p)
<no location info>: warning:
    In the type ‘R:SingV1z x_a12p r_a12q’
    Kind application error in type ‘R:SingV1z x_a12p r_a12q’
      Function kind = forall p -> V1 p -> *
      Arg kinds = [(x_a12p, *), (r_a12q, Rep Void x_a12p)]
    Fun: V1 x_a12p
         (r_a12q, Rep Void x_a12p)
<no location info>: warning:
    In the type ‘R:SingV1z x_a12p r_a12q’
    Kind application error in type ‘R:SingV1z x_a12p r_a12q’
      Function kind = forall p -> V1 p -> *
      Arg kinds = [(x_a12p, *), (r_a12q, Rep Void x_a12p)]
    Fun: V1 x_a12p
         (r_a12q, Rep Void x_a12p)
*** Offending Program ***

<elided>

$csTo_a12n :: forall x (r :: Rep Void x). Sing r -> Sing (To r)
[LclId,
 Arity=1,
 Str=<L,U>x,
 Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
         WorkFree=True, Expandable=True,
         Guidance=ALWAYS_IF(arity=1,unsat_ok=True,boring_ok=True)}]
$csTo_a12n
  = \ (@ x_a12p)
      (@ (r_a12q :: Rep Void x_a12p))
      (x_aZ8 :: Sing r_a12q) ->
      case x_aZ8
           `cast` ((Sing
                      (D:R:RepVoid[0] <x_a12p>_N) <r_a12q>_N)_R ; D:R:SingV1z0[0]
                                                                      <x_a12p>_N <r_a12q>_N
                   :: (Sing r_a12q :: *) ~R# (R:SingV1z x_a12p r_a12q :: *))
      of nt_s13T {
      }

GHC 8.0.2 does not appear to suffer from this Core Lint error.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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Operating system
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