[GHC] #14066: Skolem escape at the kind level

31 Jul 2017


      #14066: Skolem escape at the kind level
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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:
-------------------------------------+-------------------------------------
 This program should be rejected
 {{{
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE KindSignatures #-}

 module Foo2 where

 import Data.Kind ( Type )

 import Data.Type.Equality

 import Data.Proxy

 import GHC.Exts


 data SameKind :: k -> k -> Type


 f (x :: Proxy a) = let g :: forall k (b :: k). SameKind a b

                        g = undefined

                    in

                    ()
 }}}
 But

 * 8.0 rejects it, with an unhelpful message

 * 8.2 accepts it, and the result satisfies Core Lint

 * HEAD accepts it, and produces a Core Lint Error

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14066
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