Unexpected type application in default declaration

All of these work:

{-# 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:

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.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	RyanGlScott, goldfire, simonpj
Operating system
Architecture