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

[GHC]

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

 {{{#!hs
 > 0b101010
 > 0o52
 > 0x2A
 }}}

 Currently, the parser/lexer reads digits from the input as long as they
 are valid for the specified radix. All subsequent digits are interpreted
 as a new, separate token.

 If the user uses a digit which isn't valid for the radix, it may be
 reported with a non-obvious error message, or interpreted in surprising
 ways:

 {{{#!hs
 > :t 0o567
 0o576 :: Num a => a
 > :t 0o5678
 0o5678 :: (Num (a -> t), Num a) => t
 Prelude> :t 0x1bfah

 <interactive>:1:7: Not in scope: ‘h’
 > replicate 0o5678
  [8,8,8,8,8,8,8...
 }}}

 We suggest warning the user when a literal of this sort is written, while
 respecting any other error messages and the original behaviour.

 More specifically, the parser or lexer would give a warning if a token
 starting with an alphanumeric character is found immediately after a
 numeric literal, without a blank between them.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9573>
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