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

[GHC]

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

 I think you're right Simon. There is something wrong with the annotations
 though, firstly both the Missing elements have the same SrcSpan `L
 tests/examples/Tuple.hs:3:24`.

 {{{
  ((tests/examples/Tuple.hs:3:8, AnnComma),
    [tests/examples/Tuple.hs:3:9]),
   ((tests/examples/Tuple.hs:3:11-17, AnnComma),
    [tests/examples/Tuple.hs:3:18]),
   ((tests/examples/Tuple.hs:3:20-22, AnnComma),
    [tests/examples/Tuple.hs:3:23]),
   ((tests/examples/Tuple.hs:3:24, AnnComma),
    [tests/examples/Tuple.hs:3:24]),
 }}}

 Here are the relevant annotations. Notice how the first three commas are
 associated with the preceding item (much like they are for lists) but the
 last one is associated directly with the `Missing`. As both `Missing`s
 have the same SrcSpan then this is causing duplicated output in `ghc-
 exactprint`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10537#comment:2>
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