[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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Changes (by RyanGlScott):

 * status:  closed => new
 * resolution:  fixed =>
 * blockedby:   => 12045
 * milestone:  8.4.1 => 8.8.1


Comment:

 I wonder if the hack we implemented to fix this issue originally could be
 made substantially simpler with visible kind applications (which should
 [https://phabricator.haskell.org/D5229 be available soon-ish]). With VKAs,
 we could instead generate the following code:

 {{{#!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'll reopen this ticket to keep track of this idea.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14579#comment:12>
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