[GHC] #16356: Unexpected type application in default declaration

[GHC]

#16356: Unexpected type application in default declaration
-------------------------------------+-------------------------------------
           Reporter:  int-index      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 All of these work:

 {{{#!hs
 {-# LANGUAGE PolyKinds, TypeApplications, TypeFamilies, FlexibleInstances
 #-}

 import Data.Kind (Type)

 data B (a :: k)

 type family FClosed :: k -> Type where
   FClosed @k = B @k

 type family FOpen :: k -> Type
 type instance FOpen @k = B @k

 class FAssocClass k where
   type FAssoc :: k -> Type

 instance FAssocClass k where
   type FAssoc @k = B @k
 }}}

 This one doesn't:

 {{{#!hs
 class FAssocDefaultClass k where
   type FAssocDefault :: k -> Type
   type FAssocDefault @k = B @k
 }}}

 {{{
 A.hs:21:23: error:
     Unexpected type application @k
     In the default declaration for ‘FAssocDefault’
    |
 21 |   type FAssocDefault @k = B @k
    |
 }}}

 So, what are the rules of the game? Let's fix this and document in the
 User's Guide.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16356>
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