[GHC] #9573: Add warning for invalid digits in integer literals

[GHC]

#9573: Add warning for invalid digits in integer literals
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              Reporter:  vlopez      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.9
  (Parser)                           |         Keywords:  parsing, integer,
            Resolution:              |  octal, binary, hexadecimal,
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Difficult (2-5
       Type of failure:              |  days)
  None/Unknown                       |       Blocked By:
             Test Case:              |  Related Tickets:
              Blocking:              |
Differential Revisions:              |
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Comment (by vlopez):

 Here's a partial patch (attachment `invalid-digit-warning-6c9246.patch`),
 which gives a warning in most cases.

 However, the feature not very useful when the user for making sense of a
 type error, as GHC only seems to display warnings when the module can be
 compiled correctly.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9573#comment:1>
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