`Language.Haskell.TH.Syntax.reify` returns * rather than Constraint

This module is sufficient to demonstrate the mistake:

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TemplateHaskell #-}
module Bug where

import Data.Kind
import Data.Proxy
import Language.Haskell.TH

foo :: forall (k :: Constraint). Proxy k
foo = Proxy

return [] -- delimit declaration groups

main :: IO ()
main = putStrLn $(do
  VarI _ ty _ <- reify 'foo
  let p = pprint ty
  [| p |])

Running this in GHC 8.0.2 correctly prints out forall (k_0 :: Constraint) . Data.Proxy.Proxy k_0, but GHC 8.2.2 and 8.4.2 both print forall (k_0 :: *) . Data.Proxy.Proxy k_0.

Trac metadata

Trac field	Value
Version	8.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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