[GHC] #12610: Emit tab warning promptly

[GHC]

#12610: Emit tab warning promptly
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.0.2
          Component:  Compiler       |           Version:  8.0.1
  (Parser)                           |
           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:
-------------------------------------+-------------------------------------
 {{{#!hs
 p = let  x = 4
 [tab]y = 5
     in 12
 }}}

 produces `error: parse error on input ‘y’` and nothing else. The problem
 is caused by a tab appearing to the user to line up `x` with `y` but
 actually placing `y` in the wrong column. The user typically will scratch
 their head and then post a question on StackOverflow. There they'll be
 told to use `-fwarn-tabs`, but ''that option won't help'' (and it's
 probably already on) because GHC will exit with a parse error before it
 gets around to giving the warning. I suspect the solution is probably to
 emit the tab warning in the lexer, but one of the parsing gurus will
 probably know better.

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