GHC HEAD-only Core Lint error with unboxed equality (Non-CoVar has coercion type)

module Foo where

import Language.Haskell.TH.Lib
import Language.Haskell.TH.Syntax

ueqT :: Q Type
ueqT = conT $ mkNameG_tc "ghc-prim" "GHC.Prim" "~#"

{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TemplateHaskell #-}
module Bug where

import Data.Kind
import Foo (ueqT)

data JankyEquality :: Type -> Type -> Type where
  Jank :: $ueqT a b -> JankyEquality a b

unJank :: JankyEquality a b -> $ueqT a b
unJank (Jank x) = x

$ ~/Software/ghc5/inplace/bin/ghc-stage2 --interactive Bug.hs -dcore-lint
GHCi, version 8.7.20190115: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 2] Compiling Foo              ( Foo.hs, interpreted )
[2 of 2] Compiling Bug              ( Bug.hs, interpreted )
*** Core Lint errors : in result of Desugar (before optimization) ***
<no location info>: warning:
    In a case alternative: (Jank x_a4vR :: a_a4wC ~# b_a4wD)
    Non-CoVar has coercion type x_a4vR :: a_a4wC ~# b_a4wD
*** Offending Program ***

<elided>

unJank :: forall a b. JankyEquality a b -> a ~# b
[LclIdX]
unJank
  = \ (@ a_a4wC)
      (@ b_a4wD)
      (ds_d4wN :: JankyEquality a_a4wC b_a4wD) ->
      case ds_d4wN of wild_00 { Jank x_a4vR -> break<0>() x_a4vR }

Note that this passes -dcore-lint on GHC 8.0.2 through 8.6.3.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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