GitLab

While looking for a testcase for another unrelated compiler bug (see D2577) I had the pleasure of encountering this specimen,

{-# LANGUAGE TypeInType #-}
{-# LANGUAGE RankNTypes #-}
module Hi where

import GHC.Exts

data Some r = Some (TYPE r -> TYPE r)

doSomething :: forall (r :: RuntimeRep). forall (a :: TYPE r). ()
            => Int -> (a -> Int) -> a -> a
doSomething n f =
    case n of
      1 -> error "hello"
      3 -> error "hello"

GHC 465c6c5d and 8.0.1 seem to let this program typecheck, but if you enable Core Lint it soon becomes clear that not all is well. Here's a sampling of the carnage,

$ ghc hi.hs -fforce-recomp -dcore-lint -O0
[1 of 1] Compiling Hi               ( hi.hs, hi.o )
*** Core Lint errors : in result of Desugar (after optimization) ***
<no location info>: warning:
    In the type ‘a_a2ON -> Int’
    Ill-kinded argument in type or kind ‘a_a2ON -> Int’
    type or kind ‘a_a2ON -> Int’ kind: TYPE r_a2OM
<no location info>: warning:
    In the type ‘a_a2ON -> a_a2ON’
    Ill-kinded argument in type or kind ‘a_a2ON -> a_a2ON’
    type or kind ‘a_a2ON -> a_a2ON’ kind: TYPE r_a2OM
<no location info>: warning:
    In the type ‘a_a2ON -> a_a2ON’
    Ill-kinded argument in type or kind ‘a_a2ON -> a_a2ON’
    type or kind ‘a_a2ON -> a_a2ON’ kind: TYPE r_a2OM
<no location info>: warning:
    In the expression: patError
                         @ 'PtrRepLifted @ (a_a2ON -> a_a2ON) "hi.hs:(12,5)-(14,24)|case"#
    Ill-kinded argument in type or kind ‘a_a2ON -> a_a2ON’
    type or kind ‘a_a2ON -> a_a2ON’ kind: TYPE r_a2OM
...