Take this file:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Bug where

import Language.Haskell.TH

$([d| f :: (forall a. a) -> Int
      f _ = undefined |])

In GHC 8.2.2, -ddump-splices behaves as you'd expect:

$ /opt/ghc/8.2.2/bin/ghci Bug.hs
GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Bug.hs:(8,3)-(9,24): Splicing declarations
    [d| f_a1zL :: (forall a_a1zM. a_a1zM) -> Int
        f_a1zL _ = undefined |]
  ======>
    f_a49Z :: (forall a_a4a0. a_a4a0) -> Int
    f_a49Z _ = undefined
Ok, one module loaded.

But in GHC 8.4.1-alpha (and HEAD), the GHC pretty-printer incorrectly leaves off the parentheses around the type (forall a. a):

$ /opt/ghc/8.4.1/bin/ghci Bug.hs
GHCi, version 8.4.0.20171222: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Bug.hs:(8,3)-(9,24): Splicing declarations
    [d| f_a1EF :: (forall a_a1EG. a_a1EG) -> Int
        f_a1EF _ = undefined |]
  ======>
    f_a4ap :: forall a_a4aq. a_a4aq -> Int
    f_a4ap _ = undefined
Ok, one module loaded.