[GHC] #14710: GHC 8.4.1-alpha allows the use of kind polymorphism without PolyKinds

[GHC]

#14710: GHC 8.4.1-alpha allows the use of kind polymorphism without PolyKinds
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.4.1-alpha1
  checker)                           |
      Resolution:                    |             Keywords:  PolyKinds
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Actually, if we decide to require the use of `TypeInType` (and not just
 `PolyKinds`) for this featurette, then checking binding sites wouldn't be
 enough. It turns out the GHC currently accepts this program without
 `TypeInType`:

 {{{#!hs
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 module Bug where

 import Data.Proxy

 f :: forall k a. Proxy (a :: k)
 f = Proxy
 }}}

 But it seems like it shouldn't, since `k` is being quantified as a type
 variable but used as a kind variable. For comparison, if you tried this
 equivalent program:

 {{{#!hs
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 module Bug where

 import Data.Proxy

 f :: forall k (a :: k). Proxy a
 f = Proxy
 }}}

 Then GHC will bleat about `TypeInType`:

 {{{
 $ ghci Bug.hs
 GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:7:21: error:
     Type variable ‘k’ used in a kind.
     Did you mean to use TypeInType?
     the type signature for ‘f’
   |
 7 | f :: forall k (a :: k). Proxy a
   |                     ^
 }}}

 So, in short: I'm even less sure about where to put this validity check :)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14710#comment:6>
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