[GHC] #15572: TH improperly converts promoted data cons in ConT

27 Aug 2018

      #15572: TH improperly converts promoted data cons in ConT
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Template       |           Version:  8.4.3
  Haskell                            |
           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:
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 If you compile the following program:

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

 import Language.Haskell.TH

 $([d| type AbsoluteUnit1 = '() |])
 $(pure [TySynD (mkName "AbsoluteUnit2") [] (ConT '())])
 }}}
 {{{
 $ /opt/ghc/8.6.1/bin/ghci Bug.hs
 GHCi, version 8.6.0.20180810: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )
 Bug.hs:8:3-33: Splicing declarations
     [d| type AbsoluteUnit1_a1HN =  '() |]
   ======>
     type AbsoluteUnit1_a4qs =  '()
 Bug.hs:9:3-54: Splicing declarations
     pure [TySynD (mkName "AbsoluteUnit2") [] (ConT '())]
   ======>
     type AbsoluteUnit2 = ()
 }}}

 You'll notice an unusual discrepancy between the two `-ddump-splices`
 logs. In the first one:

 {{{#!hs
     type AbsoluteUnit1_a4qs =  '()
 }}}

 The `'()` constructor is properly preceded with a single quote. In the
 second one, however:

 {{{#!hs
     type AbsoluteUnit2 = ()
 }}}

 `'()` incorrectly appears without a single quote! The culprit is in the
 way `Convert`
 [http://git.haskell.org/ghc.git/blob/154d4e219cc0cebbef8a845609bd63ec45fdbea6...
 handles] `ConT`:

 {{{#!hs
           ConT nm -> do { nm' <- tconName nm
                         ; mk_apps (HsTyVar noExt NotPromoted (noLoc nm'))
 tys'}
 }}}

 This code naïvely assumes that `ConT` will never contain a promoted data
 constructor name by hardcoding `NotPromoted`. We really ought to be
 checking if `nm'` is a data con `RdrName` here and using `Promoted` if so,
 and `NotPromoted` otherwise.

 Patch incoming.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15572
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