[GHC] #10537: Parser: commas_tup_tail returns spurious "Missing" value

[GHC]

#10537: Parser: commas_tup_tail returns spurious "Missing" value
-------------------------------------+-------------------------------------
              Reporter:  alanz       |             Owner:  alanz
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:  7.10.2
             Component:  Compiler    |           Version:  7.10.1
  (Parser)                           |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 Parsing

 {{{#!hs
 {-# LANGUAGE TupleSections #-}

 baz = (1, "hello", 6.5,,) 'a' (Just ())
 }}}

 Results in the following AST fragment

 {{{
 (L tests/examples/Tuple.hs:3:7-25
    (ExplicitTuple
       [ L tests/examples/Tuple.hs:3:8
           (Present
              (L tests/examples/Tuple.hs:3:8
                 (HsOverLit
                    (OverLit
                       (HsIntegral [ '1' ] 1)
                       PlaceHolder
                       (HsLit
                          (HsString
                             []
                             {abstract:FastString}))
                       PlaceHolder))))
       , L tests/examples/Tuple.hs:3:11-17
           (Present
              (L tests/examples/Tuple.hs:3:11-17
                 (HsLit
                    (HsString
                       [ '"'
                       , 'h'
                       , 'e'
                       , 'l'
                       , 'l'
                       , 'o'
                       , '"'
                       ]
                       {abstract:FastString}))))
       , L tests/examples/Tuple.hs:3:20-22
           (Present
              (L tests/examples/Tuple.hs:3:20-22
                 (HsOverLit
                    (OverLit
                       (HsFractional
                          (FL
                             [ '6' , '.' , '5' ]
                             (:% 13 2)))
                       PlaceHolder
                       (HsLit
                          (HsString
                             []
                             {abstract:FastString}))
                       PlaceHolder))))
       , L tests/examples/Tuple.hs:3:24
           (Missing PlaceHolder)
       , L tests/examples/Tuple.hs:3:24
           (Missing PlaceHolder)
       ]
 }}}

 The final `Missing PlaceHolder` is duplicated

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10537>
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