[GHC] #12546: GeneralizedNewtypeDeriving produces error messages with incorrect kind signatures

[GHC]

#12546: GeneralizedNewtypeDeriving produces error messages with incorrect kind
signatures
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           Reporter:  lexi.lambda    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Given the following program:

 {{{
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE FlexibleInstances #-}

 import Control.Monad.Reader

 newtype AppM a = AppM (ReaderT Int IO a)
   deriving (Functor, Applicative, Monad, MonadReader)
 }}}

 The `MonadReader` deriving declaration should be `MonadReader Int`. GHC
 produces the following error message:

 {{{
 • Expecting one more argument to ‘MonadReader’
   Expected kind ‘* -> Constraint’,
     but ‘MonadReader’ has kind ‘* -> (* -> *) -> Constraint’
 • In the newtype declaration for ‘AppM’
 }}}

 This error message is confusing to me. The kind of `MonadReader` is `* ->
 (* -> *) -> Constraint`, as the error message states, which makes sense.
 However, the error message states that it expects kind `* -> Constraint`,
 despite the fact that `MonadReader Int` is actually of kind `(* -> *) ->
 Constraint`.

 ,,(This description is adapted from
 [http://stackoverflow.com/q/39172590/465378 this Stack Overflow
 question].),,

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12546>
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