[GHC] #12610: Emit tab warning promptly

[GHC]

#12610: Emit tab warning promptly
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                Owner:  dalaing
            Type:  bug               |               Status:  patch
        Priority:  high              |            Milestone:  8.4.1
       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):  Phab:D3584
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by dalaing):

 * status:  new => patch
 * differential:   => Phab:D3584


Comment:

 I'm not 100% sure on where we should be printing these warnings versus
 ignoring them.  I've generally gone with doing whatever happens to the
 warnings in the case of a successful pass.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12610#comment:11>
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