[GHC] #14840: QuantifiedConstraints: Can't define class alias

22 Feb 2018

      #14840: QuantifiedConstraints: Can't define class alias
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints wipT2893     |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# Language TypeInType, MultiParamTypeClasses, TypeFamilies,
 FlexibleContexts, FunctionalDependencies, AllowAmbiguousTypes,
 QuantifiedConstraints, GADTs, ConstraintKinds, KindSignatures, RankNTypes,
 FlexibleInstances, UndecidableInstances, TypeOperators #-}

 import Data.Kind

 class Reifies s a | s -> a where
   reflect :: proxy s -> a

 newtype Lift :: forall k. (Type -> Constraint) -> Type -> k -> Type where
   Lift :: a -> Lift cls a s

 data family Def :: (k -> Constraint) -> (k -> Type)

 class    (forall s a. Reifies s (Def cls a) => cls (Lift cls a s)) =>
 ReifiableConstraint cls
 instance (forall s a. Reifies s (Def cls a) => cls (Lift cls a s)) =>
 ReifiableConstraint cls
 }}}

 gives an error

 {{{
 $ ghci -ignore-dot-ghci Bug.hs
 GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /tmp/Bug.hs, interpreted )

 /tmp/Bug.hs:14:10: error:
     • Could not deduce: cls (Lift cls a s)
         arising from the superclasses of an instance declaration
       from the context: forall (s :: k) a.
                         Reifies s (Def cls a) =>
                         cls (Lift cls a s)
         bound by the instance declaration at /tmp/Bug.hs:14:10-93
       or from: Reifies s (Def cls a)
         bound by a quantified context at /tmp/Bug.hs:1:1
     • In the instance declaration for ‘ReifiableConstraint cls’
    |
 14 | instance (forall s a. Reifies s (Def cls a) => cls (Lift cls a s)) =>
 ReifiableConstraint cls
    |
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Failed, no modules loaded.
 Prelude>
 }}}

 This is a rare occasion that the class alias trick fails for me so
 ~+~yay~+~. But is it intended?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14840
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler