[GHC] #15831: DerivingVia allows bogus implicit quantification in `via` type

[GHC]

#15831: DerivingVia allows bogus implicit quantification in `via` type
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           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):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider the following code:

 {{{#!hs
 {-# LANGUAGE DerivingVia #-}
 {-# LANGUAGE PolyKinds #-}
 module Bug where

 import Data.Functor.Const (Const(..))
 import GHC.Exts (Any)

 newtype Age = MkAge Int
   deriving Eq
     via Const Int Any
 }}}

 This fails to compile with a spectacularly unhelpful error message:

 {{{
 $ /opt/ghc/8.6.1/bin/ghc -ddump-deriv Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 ==================== Derived instances ====================
 Derived class instances:
   instance GHC.Classes.Eq Bug.Age where
     (GHC.Classes.==)
       = GHC.Prim.coerce
           @((Data.Functor.Const.Const GHC.Types.Int (GHC.Types.Any ::
 k_a24l) :: TYPE GHC.Types.LiftedRep)
             -> (Data.Functor.Const.Const GHC.Types.Int (GHC.Types.Any ::
 k_a24l) :: TYPE GHC.Types.LiftedRep)
                -> GHC.Types.Bool)
           @(Bug.Age -> Bug.Age -> GHC.Types.Bool)
           (GHC.Classes.==) ::
           Bug.Age -> Bug.Age -> GHC.Types.Bool
     (GHC.Classes./=)
       = GHC.Prim.coerce
           @((Data.Functor.Const.Const GHC.Types.Int (GHC.Types.Any ::
 k_a24l) :: TYPE GHC.Types.LiftedRep)
             -> (Data.Functor.Const.Const GHC.Types.Int (GHC.Types.Any ::
 k_a24l) :: TYPE GHC.Types.LiftedRep)
                -> GHC.Types.Bool)
           @(Bug.Age -> Bug.Age -> GHC.Types.Bool)
           (GHC.Classes./=) ::
           Bug.Age -> Bug.Age -> GHC.Types.Bool


 Derived type family instances:



 Bug.hs:9:12: error:
     The exact Name ‘k’ is not in scope
       Probable cause: you used a unique Template Haskell name (NameU),
       perhaps via newName, but did not bind it
       If that's it, then -ddump-splices might be useful
   |
 9 |   deriving Eq
   |            ^^

 Bug.hs:9:12: error:
     The exact Name ‘k’ is not in scope
       Probable cause: you used a unique Template Haskell name (NameU),
       perhaps via newName, but did not bind it
       If that's it, then -ddump-splices might be useful
   |
 9 |   deriving Eq
   |            ^^

 Bug.hs:9:12: error:
     The exact Name ‘k’ is not in scope
       Probable cause: you used a unique Template Haskell name (NameU),
       perhaps via newName, but did not bind it
       If that's it, then -ddump-splices might be useful
   |
 9 |   deriving Eq
   |            ^^

 Bug.hs:9:12: error:
     The exact Name ‘k’ is not in scope
       Probable cause: you used a unique Template Haskell name (NameU),
       perhaps via newName, but did not bind it
       If that's it, then -ddump-splices might be useful
   |
 9 |   deriving Eq
   |            ^^
 }}}

 There are two things that are strange here:

 * Notice that in the derived `Eq` instance, there are references to
 `(GHC.Types.Any :: k_a24l)`, where `k_a24l` is completely free! This
 should never happen, and is almost surely the cause of the resulting
 volley of errors.
 * It's quite odd that we didn't reject this `deriving` clause outright
 //before// generating the derived code. In fact, if we explicitly mention
 the kind `k`:

 {{{#!hs
 newtype Age = MkAge Int
   deriving Eq
     via Const Int (Any :: k)
 }}}

   //Then// it's rejected properly:

 {{{
 $ /opt/ghc/8.6.1/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:9:12: error:
     Type variable ‘k’ is bound in the ‘via’ type ‘Const Int (Any :: k)’
     but is not mentioned in the derived class ‘Eq’, which is illegal
   |
 9 |   deriving Eq
   |            ^^
 }}}

   Something about implicit quantification must be sneaking by this
 validity check.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15831>
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