Misleading "Kind mis-match on LHS of default declaration" error

This program fails to typecheck, unsurprisingly:

{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where

class C a where
  type T a :: k
  type T a = Int

But the error message it gives threw me for a loop initially:

$ /opt/ghc/8.2.1/bin/ghci Bug.hs
GHCi, version 8.2.0.20170704: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:7:3: error:
    • Kind mis-match on LHS of default declaration for ‘T’
    • In the default type instance declaration for ‘T’
      In the class declaration for ‘C’
  |
7 |   type T a = Int
  |   ^^^^^^^^^^^^^^
Failed, 0 modules loaded.

The LHS of the default declaration is perfectly fine - the real source of the error is the RHS!

Trac metadata

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