[GHC] #12668: Program that fails Core Lint terribly

6 Oct 2016


      #12668: Program that fails Core Lint terribly
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 While looking for a testcase for another unrelated compiler bug (see
 Phab:D2577) I had the pleasure of encountering this specimen,

 {{{#!hs
 {-# 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 465c6c5d15f8fb54afb78408f3a79e75e74d2cd4 seems 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
 ...
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12668
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