-ddump-splices omits required parentheses around quantified constraints

If you compile this program with -ddump-splices:

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

import Language.Haskell.TH

data Foo x = MkFoo x

$([d| f :: (forall a. Eq (Foo a)) => Foo x -> Foo x -> Bool
      f = (==)
    |])

You'll notice something fishy:

$ /opt/ghc/8.6.1/bin/ghci Bug.hs
GHCi, version 8.6.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Bug.hs:(10,3)-(12,6): Splicing declarations
    [d| f_a1Ia ::
          (forall a_a1Ic. Eq (Foo a_a1Ic)) =>
          Foo x_a1Ib -> Foo x_a1Ib -> Bool
        f_a1Ia = (==) |]
  ======>
    f_a5tj ::
      forall a_a5tk. Eq (Foo a_a5tk) => Foo x_a5ti -> Foo x_a5ti -> Bool
    f_a5tj = (==)
Ok, one module loaded.

The signature for f gets pretty-printed as:

f_a5tj :: forall a_a5tk. Eq (Foo a_a5tk) => Foo x_a5ti -> Foo x_a5ti -> Bool

Which is just plain wrong—there is a missing set of parentheses around the quantified constraint forall a_a5tk. Eq (Foo a_a5tk).

Patch incoming.

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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