[GHC] #13257: out-of-range warnings for negative literals, without -XNegativeLiterals

[GHC]

#13257: out-of-range warnings for negative literals, without -XNegativeLiterals
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           Reporter:  rwbarton       |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 By default (without `NegativeLiterals`) a negated literal `-1` is
 desugared as `negate (fromInteger 1)`. Hence the existing out-of-range
 literal warning doesn't trigger for `-1 :: Word`. But we could also
 recognize the pattern `negate (fromInteger lit)`, and give a warning when
 `-lit` is out-of-range (for `Word`, this is whenever `lit` is positive).

 Actually this could apply to signed integer types too: if we recognized
 `negate (fromInteger lit)` in preference to `fromInteger lit`, we could
 correctly not warn about `-128 :: Int8`, even when `NegativeLiterals` is
 not enabled.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13257>
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