[GHC] #14332: Deriving clauses can have forall types

6 Oct 2017

      #14332: Deriving clauses can have forall types
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:  deriving       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC accepts
  Unknown/Multiple                   |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I made a horrifying discovery today: GHC accepts this code!

 {{{#!hs
 {-# LANGUAGE DeriveAnyClass #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE RankNTypes #-}
 {-# OPTIONS_GHC -ddump-deriv #-}
 module Bug1 where

 class C a b

 data D a = D deriving ((forall a. C a))
 }}}
 {{{
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug1             ( Bug.hs, interpreted )

 ==================== Derived instances ====================
 Derived class instances:
   instance Bug1.C a1 (Bug1.D a2) where

 Derived type family instances:

 Ok, 1 module loaded.
 }}}

 It gets even worse with this example:

 {{{#!hs
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeInType #-}
 {-# OPTIONS_GHC -ddump-deriv -fprint-explicit-kinds #-}
 module Bug1 where

 import Data.Kind
 import GHC.Generics

 data Proxy (a :: k) = Proxy
   deriving ((forall k. (Generic1 :: (k -> Type) -> Constraint)))
 }}}
 {{{
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug1             ( Bug.hs, interpreted )

 ==================== Derived instances ====================
 Derived class instances:
   instance GHC.Generics.Generic1 k (Bug1.Proxy k) where
     GHC.Generics.from1 x_a3ip
       = GHC.Generics.M1
           (case x_a3ip of { Bug1.Proxy -> GHC.Generics.M1 GHC.Generics.U1
 })
     GHC.Generics.to1 (GHC.Generics.M1 x_a3iq)
       = case x_a3iq of {
           (GHC.Generics.M1 GHC.Generics.U1) -> Bug1.Proxy }

 Derived type family instances:
   type GHC.Generics.Rep1 k_a2mY (Bug1.Proxy k_a2mY) = GHC.Generics.D1
                                                         k_a2mY
 ('GHC.Generics.MetaData
                                                            "Proxy" "Bug1"
 "main" 'GHC.Types.False)
                                                         (GHC.Generics.C1
                                                            k_a2mY
 ('GHC.Generics.MetaCons
                                                               "Proxy"
 'GHC.Generics.PrefixI
 'GHC.Types.False)
 (GHC.Generics.U1 k_a2mY))

 Ok, 1 module loaded.
 }}}

 In this example, the `forall`'d `k` from the `deriving` clause is
 discarded and then unified with the `k` from `data Proxy (a :: k)`.

 All of this is incredibly unsettling. We really shouldn't be allowing
 `forall` types in `deriving` clauses in the first place.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14332
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