[GHC] #12610: Emit tab warning promptly

[GHC]

#12610: Emit tab warning promptly
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        Reporter:  dfeuer            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.0.1
  (Parser)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Actually, the problem ''seems'' to be that the lexer adds tab warnings to
 some sort of warning log, and then `GHC.parser` only calls `getMessages`
 to retrieve a warning log summary when the parse is successful. We should
 probably change that to get at least some of the warnings in case of an
 unsuccessful parse. If it's impossible to change the type of `GHC.parser`
 between minor versions, then we could either add a function with the right
 type to use for `ghc` proper, or we could temporarily change `GHC.parser`
 to dump certain warnings into the error bucket.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12610#comment:1>
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