Recursive default associated types don't kind-generalize properly

When I say

{-# LANGUAGE PolyKinds, DataKinds, TypeFamilies, UndecidableInstances #-}

module Bug where

import Data.Proxy

class PEnum (kproxy :: KProxy a) where
  type ToEnum (x :: a) :: Bool
  type ToEnum x = TEHelper

type TEHelper = ToEnum Int

I get

/Users/rae/temp/Bug.hs:11:24:
    The first argument of ‘ToEnum’ should have kind ‘a’,
      but ‘Int’ has kind ‘*’
    In the type ‘ToEnum Int’
    In the type declaration for ‘TEHelper’

I believe my original code should kind-check, as ToEnum should be applicable to any kind.

My guess is that this sort of recursion isn't properly accounted for in the kind-checking algorithm in !TcTyClsDecls, and that we kind-check TEHelper before ToEnum is kind-generalized.

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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