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From: "darklythinking at yahoo dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/98158] New: Gcc generates warning about its own generated
 move assignment operator
Date: Sun, 06 Dec 2020 01:21:06 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98158

            Bug ID: 98158
           Summary: Gcc generates warning about its own generated move
                    assignment operator
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: darklythinking at yahoo dot com
  Target Milestone: ---

Version:
$ gcc --version
gcc (Gentoo 10.2.0-r3 p4) 10.2.0

System:
The bug doesn't happen if no arch is specified but with -march=3Dskylake or
-march=3Dznver2 the warning appears(and probably others but those are the 2=
 I
tested)

Code:
#include <string>
#include <cstdint>

struct test
{
        std::string a;
        std::uint8_t b[16];
        std::uint8_t c[16];
};

test function(test blah)
{
        blah =3D {};
        return blah;
}

int main(int argc, char *argv[])
{
        return 0;
}

Output:
$ g++ -O3 -march=3Dskylake test.cpp
In member function =E2=80=98test& test::operator=3D(test&&)=E2=80=99,
    inlined from =E2=80=98test function(test)=E2=80=99 at test.cpp:13:10:
test.cpp:4:8: warning: writing 32 bytes into a region of size 16
[-Wstringop-overflow=3D]
    4 | struct test
      |        ^~~~
test.cpp: In function =E2=80=98test function(test)=E2=80=99:
test.cpp:7:15: note: at offset 0 to object =E2=80=98test::b=E2=80=99 with s=
ize 16 declared here
    7 |  std::uint8_t b[16];
      |               ^


Note that the warning actually happens in a move assignment operator genera=
ted
by gcc itself.

The warning also appears if std::vector is used instead of the std::string =
but
I wasn't able to replace it with a struct or static array of any size and
reproduce the bug that way.=