[GHC] #14579: GeneralizedNewtypeDeriving produces ambiguously-kinded code

[GHC]

#14579: GeneralizedNewtypeDeriving produces ambiguously-kinded code
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  deriving/should_compile/T14579
      Blocked By:  12045             |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4264
       Wiki Page:                    |
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Comment (by mnguyen):

 >
 > {{{#!hs
 > {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 > {-# LANGUAGE TypeInType #-}
 > module Bug where
 >
 > import Data.Kind
 > import Data.Proxy
 >
 > newtype Wat (x :: Proxy (a :: Type)) = MkWat (Maybe a)
 >   deriving Eq
 >
 > newtype Glurp a = MkGlurp (Wat ('Proxy :: Proxy a))
 >
 > instance Eq a => Eq (Glurp a) where
 >   (==) = coerce @(Wat ('Proxy @a) -> Wat ('Proxy @a) -> Bool)
 >                 @(Glurp a         -> Glurp a         -> Bool)
 >                 (==)
 > }}}
 >
 I try this with my current VKA and it fails

 {{{#!hs
 T14579a.hs:15:32: error: Not in scope: type variable ‘a’

 T14579a.hs:15:51: error: Not in scope: type variable ‘a’

 T14579a.hs:16:25: error: Not in scope: type variable ‘a’

 T14579a.hs:16:44: error: Not in scope: type variable ‘a’
 }}}
 ???

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14579#comment:16>
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