[GHC] #14916: Missing checks when deriving special classes

[GHC]

#14916: Missing checks when deriving special classes
-------------------------------------+-------------------------------------
        Reporter:  kosmikus          |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:  Deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4501
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * status:  new => patch
 * differential:   => Phab:D4501


Comment:

 Luckily, fixing this is quite straightforward by using
 `checkValidInstHead`, which is what Phab:D4501 accomplishes.

 One thing that's interesting about `checkValidInstHead` is that is also
 does validity checks for `FlexibleInstances` and `MultiParamTypeClasses`.
 This means that using `checkValidInstHead`, unaltered, would result in
 this program, which is currently accepted by GHC:

 {{{#!hs
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 module T where

 import Control.Monad.Reader

 newtype MyReader a = MyReader (Int -> a)
   deriving ( Functor, Applicative, Monad
            , MonadReader Int )
 }}}

 Being rejected due to not enabling `FlexibleInstances` or
 `MultiParamTypeClasses`, since it generates `instance MonadReader Int
 MyReader`.

 I decided to err on the side of avoiding unnecessary breakage and tweaked
 `checkValidInstHead` so as to disable these checks for `deriving` clauses
 (just as we do for `SPECIALISE instance` pragmas today).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14916#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler