#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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler