problem with splicing type into constraint

Consider the following two-module example. (as gist: https://gist.github.com/int-e/a666991423c10150bd99dd0e874d6150)

{-# LANGUAGE TemplateHaskell #-}
module A where

mkFoo tyQ = [d|
    foo :: a ~ $(tyQ) => a
    foo = undefined
|]

{-# LANGUAGE TemplateHaskell, GADTs #-}
module B where

import A

mkFoo [t| Int -> Int |]

This loads fine in ghc-8.4.2, but with ghc-8.6.1 and current head (commit 23956b2a), the splice goes wrong:

$ ghci B.hs
GHCi, version 8.6.1: http://www.haskell.org/ghc/  :? for help
[1 of 2] Compiling A                ( A.hs, interpreted )
[2 of 2] Compiling B                ( B.hs, interpreted )

B.hs:7:1: error:
    • Expected a type, but ‘a_a4uD ~ Int’ has kind ‘Constraint’
    • In the type signature: foo :: a_a4uD ~ Int -> Int => a_a4uD
  |
7 | mkFoo [t| Int -> Int |]
  | ^^^^^^^^^^^^^^^^^^^^^^^
Failed, one module loaded.
*A> 

As a workaround one can define a type alias for the Int -> Int type.