[GHC] #8856: ScopedTypeVariables & PolyKinds lead to weird error message

6 Mar 2014

      #8856: ScopedTypeVariables & PolyKinds lead to weird error message
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       Reporter:  goldfire          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.1-rc2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 When I try to compile

 {{{
 {-# LANGUAGE ScopedTypeVariables, RankNTypes, PolyKinds #-}

 module Bug where

 import Data.Proxy

 foo = (undefined :: Proxy a) :: forall a. Proxy a
 }}}

 I get

 {{{
 Bug.hs:7:27:
     Kind variable ‘k’ used as a type
     In an expression type signature: Proxy a
     In the expression: (undefined :: Proxy a) :: forall a. Proxy a
     In an equation for ‘foo’:
         foo = (undefined :: Proxy a) :: forall a. Proxy a
 }}}

 This is all the more odd given that `k` does not appear in my code!

 This bug happens both in HEAD and in 7.8.1 RC 2.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8856
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