[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
      Resolution:                    |             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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Description changed by lexi.lambda:

@@ -3,1 +3,1 @@
- {{{
+ {{{#!haskell

New description:

 Given the following program:

 {{{#!haskell
 {-# 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].),,

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12546#comment:1>
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