[Bug c/64440] -Wdiv-by-zero false negative on const variables

["chengniansun at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64440

--- Comment #2 from Chengnian Sun <chengniansun at gmail dot com> ---

(In reply to Andrew Pinski from comment #1)
> In C, const int is not a constant expression and -Wdiv-by-zero only warns
> about integer constant expressions.

Thanks for your reply. It seems GCC sometimes does consider "const int" for
other types of warnings (but not for -Wdiv-by-zero). See the following, with
-O3, GCC warns that the left shift count is negative.  

$: cat t.c
int f(int a) {
  const char c = -4;
  return a << c;
}
$: gcc-trunk -Wall -c t.c -O3
t.c: In function ‘f’:
t.c:3:12: warning: left shift count is negative
   return a << c;
            ^
$:
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From: "kariya_mitsuru at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/64441] New: A match_results returns an incorrect sub_match if the sub_match::matched is false
Date: Tue, 30 Dec 2014 09:32:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd441

            Bug ID: 64441
           Summary: A match_results returns an incorrect sub_match if the
                    sub_match::matched is false
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kariya_mitsuru at hotmail dot com

Created attachment 34362
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id4362&actioníit
g++ -v

Please see the following sample.

====================================== sample code
=====================================#include <iostream>
#include <regex>

int main()
{
    const char s[] = "abc";
    const std::regex re("(\\d+)|(\\w+)");

    std::cmatch m;
    std::regex_search(s, m, re);
    std::cout << std::boolalpha;
    for (size_t i = 0, n = m.size(); i < n; ++i) {
        auto&& sub = m[i];
        std::cout << i << ":" << sub.matched << ", str = '" << sub.str() << "',
"
            "range = [" << sub.first - s << ", " << sub.second - s << ")" <<
std::endl;
    }
}
===================================================================================================================== output ============================0:true, str = 'abc', range = [0, 3)
1:false, str = '', range = [-140734305427376, -140734305427376)
2:true, str = 'abc', range = [0, 3)
=================================================================
cf. http://melpon.org/wandbox/permlink/SBoMF5UKYYa38Y4N


The C++11 standard 28.10[re.results]/p.4 says, "Otherwise matched is false, and
members first and second point to the end of the sequence that was searched."

So, I think that the output should be

============================= output ============================0:true, str = 'abc', range = [0, 3)
1:false, str = '', range = [3, 3)
2:true, str = 'abc', range = [0, 3)
=================================================================