Template Haskell needlessly pretty-prints empty instance contexts

This code:

{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS -ddump-splices #-}
module Bug where

$([d| class Foo a
      instance Foo Int
    |])

Used to pretty-print like this on GHC 8.0.2:

Bug2.hs:(5,3)-(7,6): Splicing declarations
    [d| class Foo_a13B a_a13C
        
        instance Foo_a13B Int |]
  ======>
    class Foo_a3Jo a_a3Jp
    instance Foo_a3Jo Int

But on GHC HEAD, it prints a redundant instance context!

Bug2.hs:(5,3)-(7,6): Splicing declarations
    [d| class Foo_a1eh a_a1ei
        
        instance Foo_a1eh Int |]
  ======>
    class Foo_a4ly a_a4lz
    instance () => Foo_a4ly Int

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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