[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, this problem dates even further back than I imagined. Another
 problematic aspect of this code is that the definition of `C` alone is
 accepted:

 {{{#!hs
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# OPTIONS_GHC -ddump-deriv #-}
 module Bug where

 import Data.Coerce
 import Data.Proxy

 class C a b where
   c :: Proxy (x :: a) -> b
 }}}

 In fact, even GHC 8.0 accepts this! This appears to be a behavioral change
 from GHC 7.10.3, where this code is rejected:

 {{{
 $ /opt/ghc/7.10.3/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:10:20:
     Type variable ‘a’ used in a kind
     In the kind ‘a’
     In the type ‘Proxy (x :: a) -> b’
     In the class declaration for ‘C’
 }}}

 (My guess is that commit 6746549772c5cc0ac66c0fce562f297f4d4b80a2, `Add
 kind equalities to GHC.`, is responsible for this.)

 Really, defining `C` should require `PolyKinds` (or perhaps even
 `TypeInType`), as the type variable `a` is also used as a kind variable
 within the type of `c`.

 You don't even need type classes for this issue to surface. Here is
 another problematic definition that uses plain old functions:

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

 import Data.Proxy

 f :: forall a. a -> a
 f x = const x g
   where
     g :: Proxy (x :: a)
     g = Proxy
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14710#comment:2>
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