-ddump-splices does not parenthesize rank-n contexts correctly

Ryan discovers another -ddump-splices bug—news at 11:

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

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

$ /opt/ghc/8.4.3/bin/ghci Bug.hs -dsuppress-uniques
GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help                                                                                                                                             
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Bug.hs:(6,3)-(8,6): Splicing declarations
    [d| f :: forall a. (Show a => a) -> a
        f _ = undefined |]
  ======>
    f :: forall a. Show a => a -> a
    f _ = undefined

Notice that f's type signature is pretty-printed as forall a. Show a => a -> a, which is just wrong. Patch incoming.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.6.1

changed weight to 5

added Tbug TemplateHaskell Trac import labels

Trac metadata

Trac field	Value
Differential revisions	- → D4910

mentioned in commit 57733978

Trac metadata

Trac field	Value
Test case	- → th/T15324

added 1 deleted label

closed

Merged in a6a83d9a.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

added Pnormal label