[GHC] #12922: Kind classes compile with PolyKinds

[GHC]

#12922: Kind classes compile with PolyKinds
-------------------------------------+-------------------------------------
        Reporter:  Tritlo            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by Tritlo:

@@ -20,0 +20,1 @@
+    type (S a) + b = S (a + b)

New description:

 I was asking around on #haskell to better understand the new language
 extension, -XTypeInType, and how it is different from -XPolyKinds.

 To study it, I was working with the following small example:

 {{{
 {-# LANGUAGE TypeFamilies, GADTs, TypeOperators, DataKinds, PolyKinds #-}
 module Main where

 import Data.Kind (Type)

 -- Define a Type for the natural numbers, Zero and a successor
 data Nat = Z | S Nat

 class Addable k where
   type (a :: k) + (b :: k) :: k

 instance Addable Nat where
   type (Z + a) = a
    type (S a) + b = S (a + b)
 }}}

 (for more context, see
 https://gist.github.com/Tritlo/ce5510e80935ac398a33934ee47c7930)

 Since according to a responder on #haskell, the ability to have kind
 classes required TypeInType, and should not work with PolyKinds.

 As the documentation mentions that there could be leakage between
 PolyKinds and TypeInType and to report such leaks, I felt that I should
 report it.

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12922#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler