Core lint error with unused kind variable in data type return kind

Finally managed to trigger a core lint error with #12045 (closed)

{-# Language    RankNTypes       #-}
{-# Language    TypeApplications #-}
{-# Language    DataKinds        #-}
{-# Language    PolyKinds        #-}
{-# Language    TypeOperators    #-}
{-# Language    GADTs            #-}
{-# Options_GHC -dcore-lint      #-}

import Data.Kind

type Cat ob = ob -> ob -> Type

data (×) :: forall (cat1 :: Cat ob1) (cat2 :: Cat ob2). Cat (ob1, ob2) where
  Prod
    :: forall
         ob1  ob2
         cat1 cat2
         a1   b1
         a2   b2 u.
       cat1 a1 b1
    -> cat2 a2 b2
    -> (×) @u '(a1, a2) '(b1, b2)

log attached from running ghci -ignore-dot-ghci <file>.hs

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	mnguyen
Operating system
Architecture