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From: "jun at junz dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110204] New: Suspicous warning when compiling ranges-v3
 using GCC trunk (iteration 9223372036854775807 invokes undefined behavior)
Date: Sat, 10 Jun 2023 14:41:03 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110204

            Bug ID: 110204
           Summary: Suspicous warning when compiling ranges-v3 using GCC
                    trunk (iteration 9223372036854775807 invokes undefined
                    behavior)
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jun at junz dot org
  Target Milestone: ---

Given code:

#include <vector>
#include <map>
#include <range/v3/all.hpp>

std::map<int, int> m;

std::vector<int> foo() {
  return ranges::to<std::vector>(m | ranges::views::keys);
}

GCC trunk reports:

/opt/compiler-explorer/gcc-trunk-20230610/include/c++/14.0.0/bits/stl_itera=
tor_base_funcs.h:175:9:
warning: iteration 9223372036854775807 invokes undefined behavior
[-Waggressive-loop-optimizations]
  175 |         while (__n--)
      |         ^~~~~
/opt/compiler-explorer/gcc-trunk-20230610/include/c++/14.0.0/bits/stl_itera=
tor_base_funcs.h:175:9:
note: within this loop

while Clang and GCC 13 both say OK. I think this is a regression.
goldbolt link: https://godbolt.org/z/W33aMqr5c=