[Bug c++/100366] New: spurious warning - std::vector::clear followed by std::vector::insert(vec.end(), ...) with -O2

[mrsam--- via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100366

            Bug ID: 100366
           Summary: spurious warning - std::vector::clear followed by
                    std::vector::insert(vec.end(), ...) with -O2
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mr...@courier-mta.com
  Target Milestone: ---

I'm seeing this with both gcc 11.1 on: https://godbolt.org/z/1Wv9jj3r1

and gcc (GCC) 11.0.1 20210324 (Red Hat 11.0.1-0), below:

Compiling the following with -O2 -Wall -Werror:

#include <vector>
#include <memory>

static char UTC[4];

void func(std::vector<char> &vec)
{
        vec.clear();
        vec.insert(vec.end(), UTC, UTC+4);
}

This generates a massive complaint that boils down to:

/usr/include/c++/11/bits/stl_algobase.h:431:30: error: ‘void*
__builtin_memcpy(v
oid*, const void*, long unsigned int)’ writing 1 or more bytes into a region of 
size 0 overflows the destination [-Werror=stringop-overflow=]
  431 |             __builtin_memmove(__result, __first, sizeof(_Tp) * _Num);
      |             ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from
/usr/include/c++/11/x86_64-redhat-linux/bits/c++allocator.

...skipping 1 line
                 from /usr/include/c++/11/bits/allocator.h:46,
                 from /usr/include/c++/11/vector:64,
                 from t.C:1:

Removing vec.clear() makes this go away.

Adding vec.reserve(4) after vec.clear() makes this go away.