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From: "MikeSmith32564 at mail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/113111] New: -Werror=uninitialized is not consistent for
 optimization level 0 or -std=before-c++20
Date: Fri, 22 Dec 2023 08:15:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113111

            Bug ID: 113111
           Summary: -Werror=3Duninitialized is not consistent for
                    optimization level 0 or -std=3Dbefore-c++20
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: MikeSmith32564 at mail dot com
  Target Milestone: ---

The following snippet:



#include <string>
#include <iostream>

struct Dummy
{
    std::string val;
};

int main()
{
    Dummy d =3D
    {
        d.val =3D "random text
#########################################################"
    };

    std::cout<<d.val<<"\n";
}



when built with:
g++ -std=3Dc++20 -O0 -Wall -Werror main.cpp -o a.out
should generate a warning a.k.a. compilation error due to -Werror about try=
ing
to assign from an uninitialized variable. When -O1, -O2 or -O3 are used the
expected warning is shown:

include/c++/14.0.0/bits/basic_string.h:1084:16: error:
'd.Dummy::val.std::__cxx11::basic_string<char>::_M_string_length' is used
uninitialized [-Werror=3Duninitialized]
 1084 |       { return _M_string_length; }
      |                ^~~~~~~~~~~~~~~~

The example triggers undefined behavior when executed of course due to runn=
ing
std::string::operator=3D with the this pointer pointing to uninitialized me=
mory
but I believe a warning should be raised when building regardless of the
optimization level selected.

Strangely, standards before c++20 also do not report the warning.=