[GHC] #13199: TH-spliced class instances are pretty-printed incorrectly post-#3384

[GHC]

#13199: TH-spliced class instances are pretty-printed incorrectly post-#3384
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The HsSyn prettyprinter tests patch
 499e43824bda967546ebf95ee33ec1f84a114a7c broke the pretty-printing of
 Template Haskell-spliced class instances. You can see this for yourself by
 compiling this code with GHC HEAD:

 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# OPTIONS_GHC -ddump-splices #-}
 module Bug where

 class C a b

 $([d| instance C a (Maybe b)
     |])
 }}}

 {{{
 $ ~/Software/ghc3/inplace/bin/ghc-stage2 --interactive Bug.hs
 GHCi, version 8.1.20170126: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )
 Bug.hs:(9,3)-(10,6): Splicing declarations
     [d| instance C a_a1eC (Maybe b_a1eD) |]
   ======>
     instance C a_a4m5 Maybe b_a4m6
 }}}

 Note the nonsensical `instance C a_a4m5 Maybe b_a4m6`. For comparison,
 here is how it should be pretty-printed:

 {{{
 $ /opt/ghc/8.0.2/bin/ghci Bug.hs
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )
 Bug.hs:(9,3)-(10,6): Splicing declarations
     [d| instance C a_a13N (Maybe b_a13O) |]
   ======>
     instance C a_a3Ju (Maybe b_a3Jv)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13199>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler