Preprocessed file see attachments, and output as generated by -v -save-temps here:

$ g++ -v -o foo -O2 -std=c++20 -save-temps repro.cpp 
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/12/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 12.2.0-13' --with-bugurl=file:///usr/share/doc/gcc-12/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-12 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-12-PBog5r/gcc-12-12.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-12-PBog5r/gcc-12-12.2.0/debian/tmp-gcn/usr --enable-offload-defaulted --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.0 (Debian 12.2.0-13) 
COLLECT_GCC_OPTIONS='-v' '-o' 'foo' '-O2' '-std=c++20' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'foo-'
 /usr/lib/gcc/x86_64-linux-gnu/12/cc1plus -E -quiet -v -imultiarch x86_64-linux-gnu -D_GNU_SOURCE repro.cpp -mtune=generic -march=x86-64 -std=c++20 -O2 -fpch-preprocess -fasynchronous-unwind-tables -o foo-repro.ii
ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/12"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/12/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/12
 /usr/include/x86_64-linux-gnu/c++/12
 /usr/include/c++/12/backward
 /usr/lib/gcc/x86_64-linux-gnu/12/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-o' 'foo' '-O2' '-std=c++20' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'foo-'
 /usr/lib/gcc/x86_64-linux-gnu/12/cc1plus -fpreprocessed foo-repro.ii -quiet -dumpdir foo- -dumpbase repro.cpp -dumpbase-ext .cpp -mtune=generic -march=x86-64 -O2 -std=c++20 -version -fasynchronous-unwind-tables -o foo-repro.s
GNU C++20 (Debian 12.2.0-13) version 12.2.0 (x86_64-linux-gnu)
	compiled by GNU C version 12.2.0, GMP version 6.2.1, MPFR version 4.1.1-p1, MPC version 1.3.1, isl version isl-0.25-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++20 (Debian 12.2.0-13) version 12.2.0 (x86_64-linux-gnu)
	compiled by GNU C version 12.2.0, GMP version 6.2.1, MPFR version 4.1.1-p1, MPC version 1.3.1, isl version isl-0.25-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: b9e5387ca128f541a98a8ca5afd00294
In function ‘Vector make_vector()’,
    inlined from ‘int main()’ at repro.cpp:36:31:
repro.cpp:27:11: warning: ‘void* memset(void*, int, size_t)’ writing 128 bytes into a region of size 40 overflows the destination [-Wstringop-overflow=]
   27 |     memset(buffer.data(), 'A', new_size);
      |     ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
repro.cpp: In function ‘int main()’:
repro.cpp:36:10: note: destination object ‘actual’ of size 40
   36 |     auto actual = make_vector();
      |          ^~~~~~
COLLECT_GCC_OPTIONS='-v' '-o' 'foo' '-O2' '-std=c++20' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'foo-'
 as -v --64 -o foo-repro.o foo-repro.s
GNU assembler version 2.39.90 (x86_64-linux-gnu) using BFD version (GNU Binutils for Debian) 2.39.90.20230104
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-o' 'foo' '-O2' '-std=c++20' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'foo.'
 /usr/lib/gcc/x86_64-linux-gnu/12/collect2 -plugin /usr/lib/gcc/x86_64-linux-gnu/12/liblto_plugin.so -plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/12/lto-wrapper -plugin-opt=-fresolution=foo.res -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu --as-needed -dynamic-linker /lib64/ld-linux-x86-64.so.2 -pie -o foo /usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/Scrt1.o /usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/12/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/12 -L/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib -L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/12/../../.. foo-repro.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/12/crtendS.o /usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-v' '-o' 'foo' '-O2' '-std=c++20' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'foo.'

Created attachment 54242 [details]
precompiled file as generated by -save-temps

Started with r12-145-gd1d01a66012a93cc.

Warns from

#1  0x00000000013bc420 in warn_for_access<gimple*> (loc=2147485003, 
    func=<function_decl 0x7ffff633ad00 memset>, exp=<gimple_call 0x7ffff60938e8>, 
    opt=761, range=0x7fffffffd560, size=<integer_cst 0x7ffff63f42e8>, 
    write=true, read=false, maybe=false)
    at /home/rguenther/src/gcc-12-branch/gcc/gimple-ssa-warn-access.cc:995
(gdb) l
990         }
991
992       if (write)
993         {
994           if (tree_int_cst_equal (range[0], range[1]))
995             warned = (func
996                       ? warning_n (loc, opt, tree_to_uhwi (range[0]),
997                                    (maybe
998                                     ? G_("%qD may write %E byte into a region "
999                                          "of size %E")
(gdb) p debug_gimple_stmt (exp)
# .MEM_2 = VDEF <.MEM_23>
memset (&MEM <char[32]> [(void *)&actual], 65, 128);

on a path where actual.m_outline == nullptr

for some unknown reason we reload actual.m_outline in the loop, likely
because storing to it is thought to clobber actual.m_outline
(which is initialized from a new expression).  Note 'actual' escapes
the function via the printf call and 'new' can inspect/clobber globals.

We're also "bad" in computing points-to info because of the

    memset(buffer.data(), 'A', new_size);

which with

    char* data() {
        if (m_outline)
            return m_outline;
        return reinterpret_cast<char*>(m_inline);
    }

simply clobbers the whole object (with our points-to analysis).

Helping the compiler and doing

    auto *b = buffer.m_outline;
    for (unsigned i = 0; i < 128; ++i)
        b[i] = 0;

allows it to optimize and avoid the diagnostic.  Using buffer.m_outline
in the memset instead of buffer.data () would probably work as well.

GCC 12.3 is being released, retargeting bugs to GCC 12.4.

GCC 12.4 is being released, retargeting bugs to GCC 12.5.