Can create bindings of kind Constraint without ConstraintKind, only TypeFamilies

This code compiles with GHC 8.0.1 (in either variant, with or without the definition of ApplyCtx Int in Foo Int. I think it shouldn't according to the docs in http://ghc.readthedocs.io/en/master/glasgow_exts.html?highlight=constraintKinds#the-constraint-kind. If this is intended, I guess documenting it is acceptable.

{-# LANGUAGE TypeFamilies #-}
module Foo where

import GHC.Exts (Constraint)
class Foo a where
  type ApplyCtx a :: Constraint
  type ApplyCtx a = ()

instance Foo Int where
  type ApplyCtx Int = Show Int -- Commenting this out makes no difference.

f :: ApplyCtx Int => Int
f = 0

Googling found https://mail.haskell.org/pipermail/glasgow-haskell-users/2016-February/026151.html, but seems a different issue (and a non-bug). Also found #11715 (closed), which might be relevant (or not).

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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