[GHC] #16139: GHC confused about type synonym kind with QuantifiedConstraints

6 Jan 2019

      #16139: GHC confused about type synonym kind with QuantifiedConstraints
-------------------------------------+-------------------------------------
           Reporter:  Ashley         |             Owner:  (none)
  Yakeley                            |
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# LANGUAGE KindSignatures, RankNTypes, ConstraintKinds,
 QuantifiedConstraints #-}
 module Bug where
 import Data.Constraint

 type E (c :: * -> Constraint) = forall (a :: *). Eq a => c a
 }}}

 {{{
 Bug.hs:5:58: error:
     • Expected a type, but ‘c a’ has kind ‘Constraint’
     • In the type ‘forall (a :: *). Eq a => c a’
       In the type declaration for ‘E’
   |
 5 | type E (c :: * -> Constraint) = forall (a :: *). Eq a => c a
   |                                                          ^^^
 }}}

 Note that GHC accepts the program when the `Eq a` constraint is removed.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16139
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